Book Excerpt: Programming in Python 3

Read chapter 12 from the book Programming in Python 3: A Complete Introduction to the Python Language by Mark Summerfield.

This chapter is an excerpt from Programming in Python 3: A Complete Introduction to the Python Language , authored by Mark Summerfield, published in the Addison-Wesley Professional Developer’s Library, Dec. 2008, ISBN 0137129297, Copyright 2009 Pearson Education, Inc. For more info check out Summerfield's new Digital Short Cut: Advanced Python 3 Programming Techniques


Chapter 12: Regular Expressions

  • Python’ s Regular Expression Language
  • The Regular Expression Module

A regular expression is a compact notation for representing a collection of strings. What makes regular expressions so powerful is that a single regular expression can represent an unlimited number of strings—providing they meet the regular expression’s requirements. Regular expressions (which we will mostly call “regexes” from now on) are defined using a mini-language that is completely different from Python—but Python includes the re module through which we can seamlessly create and use regexes.1

Regexes are used for four main purposes:

  • Validation: checking whether a piece of text meets some criteria, for example, contains a currency symbol followed by digits
  • Searching: locating substrings that can have more than one form, for example, finding any of “pet.png”, “pet.jpg”, “pet.jpeg”, or “pet.svg” while avoiding “carpet.png” and similar
  • Searching and replacing: replacing everywhere the regex matches with a string, for example, finding “bicycle” or “human powered vehicle” and replacing either with “bike”
  • Splitting strings: splitting a string at each place the regex matches, for example, splitting everywhere “: ” or “=” is encountered

At its simplest a regular expression is an expression (e.g., a literal character), optionally followed by a quantifier. More complex regexes consist of any number of quantified expressions and may include assertions and may be influenced by flags.

This chapter’s first section introduces and explains all the key regular expression concepts and shows pure regular expression syntax—it makes minimal reference to Python itself. Then the second section shows how to use regular expressions in the context of Python programming, drawing on all the material covered in the earlier sections. Readers familiar with regular expressions who just want to learn how they work in Python could skip to the second section. The chapter covers the complete regex language offered by the re module, including all the assertions and flags. We indicate regular expressions in the text using bold, show where they match using underlining, and show captures using shading.

Python’s Regular Expression Language

In this section we look at the regular expression language in four subsections. The first subsection shows how to match individual characters or groups of characters, for example, match a, or match b, or match either a or b. The second subsection shows how to quantify matches, for example, match once, or match at least once, or match as many times as possible. The third subsection shows how to group subexpressions and how to capture matching text, and the final subsection shows how to use the language’s assertions and flags to affect how regular expressions work.

Characters and Character Classes

The simplest expressions are just literal characters, such as a or 5, and if no quantifier is explicitly given it is taken to be “match one occurrence”. For example, the regex tune consists of four expressions, each implicitly quantified to match once, so it matches one t followed by one u followed by one n followed by one e, and hence matches the strings tune and attuned.

Although most characters can be used as literals, some are “special characters”—these are symbols in the regex language and so must be escaped by preceding them with a backslash (\) to use them as literals. The special characters are \.^$?+*{}[]()|. Most of Python’s standard string escapes can also be used within regexes, for example, \n for newline and \t for tab, as well as hexadecimal escapes for characters using the \xHH, \uHHHH, and \UHHHHHHHH syntaxes.

In many cases, rather than matching one particular character we want to match any one of a set of characters. This can be achieved by using a character class—one or more characters enclosed in square brackets. (This has nothing to do with a Python class, and is simply the regex term for “set of characters”.) A character class is an expression, and like any other expression, if not explicitly quantified it matches exactly one character (which can be any of the characters in the character class). For example, the regex r[ea]d matches both red and radar, but not read. Similarly, to match a single digit we can use the regex [0123456789]. For convenience we can specify a range of characters using a hyphen, so the regex [0-9] also matches a digit. It is possible to negate the meaning of a character class by following the opening bracket with a caret, so [^0-9] matches any character that is not a digit.

Note that inside a character class, apart from \, the special characters lose their special meaning, although in the case of ^ it acquires a new meaning (negation) if it is the first character in the character class, and otherwise is simply a literal caret. Also, -signifies a character range unless it is the first character, in which case it is a literal hyphen.

Since some sets of characters are required so frequently, several have short-hand forms—these are shown in Table12.1.With one exception the shorthands can be used inside character sets, so for example, the regex [\dA-Fa-f] matches any hexadecimal digit. The exception is . which is a shorthand outside a character class but matches a literal . inside a character class.

Table 12.1 Character Class Shorthands

Symbol

Meaning

. 

Matches any character except newline; or any character at all with the re.DOTALL flag; or inside a character class matches a literal .

\d 

Matches a Unicode digit; or [0-9] with the re.ASCII flag

\D 

Matches a Unicode nondigit; or [^0-9] with the re.ASCII flag

\s 

Matches a Unicode whitespace; or [ \t\n\r\f\v] with the re.ASCII flag

\S 

Matches a Unicode nonwhitespace; or [^ \t\n\r\f\v] with the re.ASCII flag

\w 

Matches a Unicode “word” character; or [a-zA-Z0-9_] with the re.ASCII flag

\W 

Matches a Unicode non-“word” character; or [^a-zA-Z0-9_] with the re.ASCII flag

Quantifiers

A quantifier has the form {m,n} where m and n are the minimum and maximum times the expression the quantifier applies to must match. For example, both e{1,1}e{1,1} and e{2,2} match feel, but neither matches felt.

Writing a quantifier after every expression would soon become tedious, and is certainly difficult to read. Fortunately, the regex language supports several convenient shorthands. If only one number is given in the quantifier it is taken to be both the minimum and the maximum, so e{2} is the same as e{2,2}. And as we noted in the preceding section, if no quantifier is explicitly given, it is assumed to be one (i.e., {1,1} or {1}); therefore, ee is the same as e{1,1}e{1,1} and e{1}e{1}, so both e{2} and ee match feel but not felt.

Having a different minimum and maximum is often convenient. For example, to match travelled and traveled (both legitimate spellings), we could use either travel{1,2}ed or travell{0,1}ed. The {0,1} quantification is so often used that it has its own shorthand form, ?, so another way of writing the regex (and the one most likely to be used in practice) is travell?ed.

Two other quantification shorthands are provided: + which stands for {1,n} (“at least one”) and * which stands for {0,n} (“any number of”);in both cases n is the maximum possible number allowed for a quantifier, usually at least 32 767. All the quantifiers are shown in Table 12.2.

The + quantifier is very useful. For example, to match integers we could use \d+ since this matches one or more digits. This regex could match in two places in the string 4588.91, for example, 4588.91 and 4588.91. Sometimes typos are the result of pressing a key too long. We could use the regex bevel+ed to match the legitimate beveled and bevelled, and the incorrect bevellled. If we wanted to standardize on the one l spelling, and match only occurrences that had two or more ls, we could use bevell+ed to find them.

The * quantifier is less useful, simply because it can so often lead to unexpected results. For example, supposing that we want to find lines that contain comments in Python files, we might try searching for #*. But this regex will match any line whatsoever, including blank lines because the meaning is “match any number of #s”—and that includes none. As a rule of thumb for those new to regexes, avoid using * at all, and if you do use it (or if you use ?), make sure there is at least one other expression in the regex that has a non-zero quantifier—so at least one quantifier other than * or ? since both of these can match their expression zero times.

It is often possible to convert * uses to + uses and vice versa. For example, we could match “tasselled” with at least one l using tassell*ed or tassel+ed, and match those with two or more ls using tasselll*ed or tassell+ed.

If we use the regex \d+ it will match 136. But why does it match all the digits, rather than just the first one? By default, all quantifiers are greedy—they match as many characters as they can. We can make any quantifier nongreedy (also called minimal) by following it with a ? symbol. (The question mark has two different meanings—on its own it is a shorthand for the {0,1} quantifier, and when it follows a quantifier it tells the quantifier to be nongreedy.) For example, \d+? can match the string 136 in three different places: 136, 136, and 136. Here is another example: \d?? matches zero or one digits, but prefers to match none since it is nongreedy—on its own it suffers the same problem as * in that it will match nothing, that is, any text at all.

Table 12.2 Regular Expression Quantifiers

Syntax

Meaning

e? or e{0,1} 

Greedily match zero or one occurrence of expression e

e?? or e{0,1}? 

Nongreedily match zero or one occurrence of expression e

e+ or e{1,} 

Greedily match one or more occurrences of expression e

e+? or e{1,}? 

Nongreedily match one or more occurrences of expression e

e* or e{0,} 

Greedily match zero or more occurrences of expression e

e*? or e{0,}? 

Nongreedily match zero or more occurrences of expression e

e{m} 

Match exactly m occurrences of expression e

e{m,} 

Greedily match at least m occurrences of expression e

e{m,}? 

Nongreedily match at least m occurrences of expression e

e{,n} 

Greedily match at most n occurrences of expression e

e{,n}? 

Nongreedily match at most n occurrences of expression e

e{m,n} 

Greedily match at least m and at most n occurrencesof expression e

e{m,n}? 

Nongreedily match at least m and at most n occurrences of expression e

Nongreedy quantifiers can be useful for quick and dirty XML and HTML parsing. For example, to match all the image tags, writing <img.*> (match one “<”, then one “i”, then one “m”, then one “g”, then zero or more of any character apart from newline, then one “>”) will not work because the .* part is greedy and will match everything including the tag’s closing >, and will keep going until it reaches the last > in the entire text.

Three solutions present themselves (apart from using a proper parser). One is <img[^>]*> (match <img, then any number of non-> characters and then the tag’s closing > character), another is <img.*?> (match <img, then any number of characters, but nongreedily, so it will stop immediately before the tag’s closing >, and then the >), and a third combines both, as in <img[^>]*?>. None of them is correct, though, since they can all match <img>, which is not valid. Since we know that an image tag must have a src attribute, a more accurate regex is <img\s+[^>]*?src=\w+[^>]*?>. This matches the literal characters <img, then one or more whitespace characters, then nongreedily zero or more of anything except > (to skip any other attributes such as alt), then the src attribute (the literal characters src= then at least one “word” character), and then any other non-> characters (including none) to account for any other attributes, and finally the closing >.

Grouping and Capturing

In practical applications we often need regexes that can match any one of two or more alternatives, and we often need to capture the match or some part of the match for further processing. Also, we sometimes want a quantifier to apply to several expressions. All of these can be achieved by grouping with (), and in the case of alternatives using alternation with |.

Alternation is especially useful when we want to match any one of several quite different alternatives. For example, the regex aircraft|airplane|jet will match any text that contains “aircraft” or “airplane” or “jet”. The same thing can be achieved using the regex air(craft|plane)|jet. Here, the parentheses are used to group expressions, so we have two outer expressions, air(craft|plane) and jet. The first of these has an inner expression, craft|plane, and because this is preceded by air the first outer expression can match only “aircraft” or “airplane”.

Parentheses serve two different purposes—to group expressions and to capture the text that matches an expression. We will use the term group to refer to a grouped expression whether it captures or not, and capture and capture group to refer to a captured group. If we used the regex (aircraft|airplane|jet) it would not only match any of the three expressions, but would also capture whichever one was matched for later reference. Compare this with the regex (air(craft|plane)|jet) which has two captures if the first expression matches (“aircraft” or “airplane” as the first capture and “craft” or “plane” as the second capture), and one capture if the second expression matches (“jet”). We can switch off the capturing effect by following an opening parenthesis with ?:, so for example, (air(?:craft|plane)|jet) will have only one capture if it matches (“aircraft” or “airplane” or “jet”).

A grouped expression is an expression and so can be quantified. Like any other expression the quantity is assumed to be one unless explicitly given. For example, if we have read a text file with lines of the form key=value, where each key is alphanumeric, the regex (\w+)=(.+) will match every line that has a nonempty key and a nonempty value. (Recall that . matches anything except newlines.) And for every line that matches, two captures are made, the first being the key and the second being the value.

For example, the key=value regular expression will match the entire line topic= physical geography with the two captures shown shaded. Notice that the second capture includes some whitespace, and that whitespace before the = is not accepted. We could refine the regex to be more flexible in accepting whitespace, and to strip off unwanted whitespace using a somewhat longer version:

[ \t]*(\w+)[ \t]*=[ \t]*(.+)

This matches the same line as before and also lines that have whitespace around the = sign, but with the first capture having no leading or trailing whitespace, and the second capture having no leading whitespace. For example: topic = physical geography.

We have been careful to keep the whitespace matching parts outside the capturing parentheses, and to allow for lines that have no whitespace at all. We did not use \s to match whitespace because that matches newlines (\n) which could lead to incorrect matches that span lines (e.g., if the re.MULTILINE flag is used). And for the value we did not use \S to match nonwhitespace because we want to allow for values that contain whitespace (e.g., English sentences). To avoid the second capture having trailing whitespace we would need a more sophisticated regex; we will see this in the next subsection.

Captures can be referred to using backreferences, that is, by referring back to an earlier capture group.2 One syntax for backreferences inside regexes themselves is \i where i is the capture number. Captures are numbered starting from one and increasing by one going from left to right as each new (capturing) left parenthesis is encountered. For example, to simplistically match duplicated words we can use the regex (\w+)\s+\1 which matches a “word”, then at least one whitespace, and then the same word as was captured. (Capture number 0 is created automatically without the need for parentheses; it holds the entire match, that is, what we show underlined.) We will see a more sophisticated way to match duplicate words later.

In long or complicated regexes it is often more convenient to use names rather than numbers for captures. This can also make maintenance easier since adding or removing capturing parentheses may change the numbers but won’t affect names. To name a capture we follow the opening parenthesis with ?P<name>. For example, (?P<key>\w+)=(?P<value>.+) has two captures called "key" and "value". The syntax for backreferences to named captures inside a regex is (?P=name). For example, (?P<word>\w+)\s+(?P=word) matches duplicate words using a capture called "word".

Assertions and Flags

One problem that affects many of the regexes we have looked at so far is that they can match more or different text than we intended. For example, the regex aircraft|airplane|jet will match “waterjet” and “jetski” as well as “jet”. This kind of problem can be solved by using assertions. An assertion does not match any text, but instead says something about the text at the point where the assertion occurs.

One assertion is \b (word boundary), which asserts that the character that precedes it must be a “word” (\w) and the character that follows it must be a non“word” (\W), or vice versa. For example, although the regex jet can match twice in the text the jet and jetski are noisy, that is, the jet and jetski are noisy, the regex \bjet\b will match only once, the jet and jetski are noisy. In the context of the original regex, we could write it either as \baircraft\b|\bairplane\b|\bjet\b or more clearly as \b(?:aircraft|airplane|jet)\b, that is, word boundary, noncapturing expression, word boundary.

Many other assertions are supported, as shown in Table 12.3. We could use assertions to improve the clarity of a key=value regex, for example, by changing it to ^(\w+)=([^\n]+) and setting the re.MULTILINE flag to ensure that each key=value is taken from a single line with no possibility of spanning lines. (The flags are shown in Table 12.5 on page 460, and the syntaxes for using them are described at the end of this subsection and are shown in the next section.) And if we also want to strip leading and trailing whitespace and use named captures, the full regex becomes:

^[ \t]*(?P<key>\w+)[ \t]*=[ \t]*(?P<value>[^\n]+)(?<![ \t])

Even though this regex is designed for a fairly simple task, it looks quite complicated. One way to make it more maintainable is to include comments in it. This can be done by adding inline comments using the syntax (?#the comment), but in practice comments like this can easily make the regex even more difficult to read. A much nicer solution is to use the re.VERBOSE flag—this allows us to freely use whitespace and normal Python comments in regexes, with the one constraint that if we need to match whitespace we must either use \s or a character class such as []. Here’s the key=value regex with comments:

^[ \t]*           # start of line and optional leading whitespace
(?P<key>\w+)      # the key text
[ \t]*=[ \t]*     # the equals with optional surrounding whitespace
(?P<value>[^\n]+) # the value text
(?<![ \t])        # negative lookbehind to avoid trailing whitespace

In the context of a Python program we would normally write a regex like this inside a raw triple quoted string—raw so that we don’t have to double up the backslashes, and triple quoted so that we can spread it over multiple lines.

In addition to the assertions we have discussed so far, there are additional assertions which look at the text in front of (or behind) the assertion to see whether it matches (or does not match) an expression we specify. The expressions that can be used in lookbehind assertions must be of fixed length (so the quantifiers ?, +, and * cannot be used, and numeric quantifiers must be of a fixed size, for example, {3}).

Table 12.3 Regular Expression Assertions

Symbol

Meaning

^ 

Matches at the start; also matches after each newline with the re.MULTILINE flag

$ 

Matches at the end; also matches before each newline with the re.MULTILINE flag

\A 

Matches at the start

\b 

Matches at a “word” boundary; influenced by the re.ASCII flag—inside a character class this is the escape for the backspace character

\B 

Matches at a non-“word” boundary; influenced by the re.ASCII flag

\Z 

Matches at the end

(?=e) 

Matches if the expression e matches at this assertion but does not advance over it—called lookahead or positive lookahead

(?!e) 

Matches if the expression e does not match at this assertion and does not advance over it—called negative lookahead

 (?<=e)

Matches if the expression e matches immediately before this assertion—called positive lookbehind

(?<!e)

Matches if the expression e does not match immediately before this assertion—called negative lookbehind

In the case of the key=value regex, the negative lookbehind assertion means that at the point it occurs the preceding character must not be a space or a tab. This has the effect of ensuring that the last character captured into the "value" capture group is not a space or tab (yet without preventing spaces or tabs from appearing inside the captured text).

Let’s consider another example. Suppose we are reading a multiline text that contains the names “Helen Patricia Sharman”, “Jim Sharman”, “Sharman Joshi”, “Helen Kelly”, and so on, and we want to match “Helen Patricia”, but only when referring to “Helen Patricia Sharman”. The easiest way is to use the regex \b(Helen\s+Patricia)\s+Sharman\b. But we could also achieve the same thing using a lookahead assertion, for example, \b(Helen\s+Patricia)(?=\s+Sharman\b). This will match “Helen Patricia” only if it is preceded by a word boundary and followed by whitespace and “Sharman” ending at a word boundary.

To capture the particular variation of the forenames that is used (“Helen”, “Helen P.”, or “Helen Patricia”), we could make the regex slightly more sophisticated, for example, \b(Helen(?:\s+(?:P\.|Patricia))?)\s+(?=Sharman\b). This matches a word boundary followed by one of the forename forms—but only if this is followed by some whitespace and then “Sharman” and a word boundary.

Note that only two syntaxes perform capturing, (e) and (?P<name>e). None of the other parenthesized forms captures. This makes perfect sense for the lookahead and lookbehind assertions since they only make a statement about what follows or precedes them—they are not part of the match, but rather affect whether a match is made. It also makes sense for the last two parenthesized forms that we will now consider.

We saw earlier how we can backreference a capture inside a regex either by number (e.g., \1) or by name (e.g., (?P=name)). It is also possible to match conditionally depending on whether an earlier match occurred. The syntaxes are (?(id)yes_exp) and (?(id)yes_exp|no_exp). The id is the name or number of an earlier capture that we are referring to. If the capture succeeded the yes_exp will be matched here. If the capture failed the no_exp will be matched if it is given.

Let’s consider an example. Suppose we want to extract the filenames referred to by the src attribute in HTML img tags. We will begin just by trying to match the src attribute, but unlike our earlier attempt we will account for the three forms that the attribute’s value can take: single quoted, double quoted, and unquoted. Here is an initial attempt: src=(["'])([^"'>]+)\1. The ([^"'>]+) part captures a greedy match of at least one character that isn’t a quote or >. This regex works fine for quoted filenames, and thanks to the \1 matches only when the opening and closing quotes are the same. But it does not allow for unquoted filenames. To fix this we must make the opening quote optional and therefore match only it if it is present. Here is the revised regex: src=(["'])?([^"'>]+)(?(1)\1). We did not provide a no_exp since there is nothing to match if no quote is given. Now we are ready to put the regex in context—here is the complete img tag regex using named groups and comments:

<img\s+              # start of the tag
[^>]*?               # any attributes that precede the src
src=                 # start of the src attribute
(?P<quote>["'])?     # optional opening quote
(?P<image>[^"'>]+)   # image filename
(?(quote)(?P=quote)) # closing quote (matches opening quote if given)
[^>]*?               # any attributes that follow the src
>                    # end of the tag

The filename capture is called "image" (which happens to be capture number 2).

Of course, there is a simpler but subtler alternative: src=(["']?)([^"'>]+)\1. Here, if there is a starting quote character it is captured into capture group 1 and matched after the nonquote characters. And if there is no starting quote character, group 1 will still match—an empty string since it is completely optional (its quantifier is zero or one), in which case the backreference will also match an empty string.

The final piece of regex syntax that Python’s regular expression engine offers is a means of setting the flags. Usually the flags are set by passing them as additional parameters when calling the re.compile() function, but sometimes it is more convenient to set them as part of the regex itself. The syntax is simply (?flags) where flags is one or more of a (the same as passing re.ASCII), i (re.IGNORECASE), m (re.MULTILINE), s (re.DOTALL), and x (re.VERBOSE).3 If the flags are set this way they should be put at the start of the regex; they match nothing, so their effect on the regex is only to set the flags.

The Regular Expression Module

The re module provides two ways of working with regexes. One is to use the functions listed in Table 12.4, where each function is given a regex as its first argument. Each function converts the regex into an internal format—a process called compiling—and then does its work. This is very convenient for one-off uses, but if we need to use the same regex repeatedly we can avoid the cost of compiling it at each use by compiling it once using the re.compile() function. We can then call methods on the compiled regex object as many times as we like. The compiled regex methods are listed in Table 12.6.

match = re.search(r"#[\dA-Fa-f]{6}\b", text)

This code snippet shows the use of an re module function. The regex matches HTML-style colors (such as #C0C0AB). If a match is found the re.search() function returns a match object; otherwise, it returns None. The methods provided by match objects are listed in Table 12.7

If we were going to use this regex repeatedly, we could compile it once and then use the compiled regex whenever we needed it:

color_re = re.compile(r"#[\dA-Fa-f]{6}\b")
match = color_re.search(text)

As we noted earlier, we use raw strings to avoid having to escape backslashes. Another way of writing this regex would be to use the character class [\dA-F] and pass the re.IGNORECASE flag as the last argument to the re.compile() call, or to use the regex (?i)#[\dA-F]{6}\b which starts with the ignore case flag.

If more than one flag is required they can be combined using the OR operator (|), for example, re.MULTILINE|re.DOTALL, or (?ms) if embedded in the regex itself.

We will round off this section by reviewing some examples, starting with some of the regexes shown in earlier sections, so as to illustrate the most commonly used functionality that the re module provides. Let’s start with a regex to spot duplicate words:

double_word_re = re.compile(r"\b(?P<word>\w+)\s+(?P=word)(?!\w)",
                            re.IGNORECASE)
for match in double_word_re.finditer(text):
       print("{0} is duplicated".format(match.group("word")))

The regex is slightly more sophisticated than the version we made earlier. It starts at a word boundary (to ensure that each match starts at the beginning of a word), then greedily matches one or more “word” characters, then one or more whitespace characters, then the same word again—but only if the second occurrence of the word is not followed by a word character.

If the input text was “win in vain”, without the first assertion there would be one match and two captures: win in vain. The use of the word boundary assertion ensures that the first word matched is a whole word, so we end up with no match or capture since there is no duplicate word. Similarly, if the input text was “one and and two let’s say”, without the last assertion there would be two matches and two captures: one and and two let's say. The use of the lookahead assertion means that the second word matched is a whole word, so we end up with one match and one capture: one and and two let's say.

The for loop iterates over every match object returned by the finditer() method and we use the match object’s group() method to retrieve the captured group’s text. We could just as easily (but less maintainably) have used group(1)—in which case we need not have named the capture group at all and just used the regex (\w+)\s+\1(?!\w). Another point to note is that we could have used a word boundary \b at the end, instead of (?!\w).

Another example we presented earlier was a regex for finding the filenames in HTML image tags. Here is how we would compile the regex, adding flags so that it is not case-sensitive, and allowing us to include comments:

image_re = re.compile(r"""
               <img\s+               # start of tag
               [^>]*?                # non-src attributes
               src=                  # start of src attribute
               (?P<quote>["'])?      # optional opening quote
               (?P<image>[^"'>]+)    # image filename
               (?(quote)(?P=quote))  # closing quote
               [^>]*?                # non-src attributes
               >                     # end of the tag
               """, re.IGNORECASE|re.VERBOSE)
image_files = []
for match in image_re.finditer(text):
    image_files.append(match.group("image"))

Again we use the finditer() method to retrieve each match and the match object’s group() function to retrieve the captured texts. Since the case insensitivity applies only to img and src, we could drop the re.IGNORECASE flag and use [Ii][Mm][Gg] and [Ss][Rr][Cc] instead. Although this would make the regex less clear, it might make it faster since it would not require the text being matched to be set to upper-(or lower-) case—but it is likely to make a difference only if the regex was being used on a very large amount of text.

One common task is to take an HTML text and output just the plain text that it contains. Naturally we could do this using one of Python’s parsers, but a simple tool can be created using regexes. There are three tasks that need to be done: delete any tags, replace entities with the characters they represent, and insert blank lines to separate paragraphs. Here is a function (taken from the html2text.py program) that does the job:

def html2text(html_text):
    def char_from_entity(match):
        code = html.entities.name2codepoint.get(match.group(1), 0xFFFD)
        return chr(code)
    text = re.sub(r"<!--(?:.|\n)*?-->", "", html_text)          #1
    text = re.sub(r"<[Pp][^>]*?(?!</)>", "\n\n", text)          #2
    text = re.sub(r"<[^>]*?>", "", text)                        #3
    text = re.sub(r"&#(\d+);", lambda m: chr(int(m.group(1))), text)
    text = re.sub(r"&([A-Za-z]+);", char_from_entity, text)     #5
    text = re.sub(r"\n(?:[ \xA0\t]+\n)+", "\n", text)           #6
    return re.sub(r"\n\n+", "\n\n", text.strip())               #7

The first regex, <!--(?:.|\n)*?-->, matches HTML comments, including those with other HTML tags nested inside them. The re.sub() function replaces as many matches as it finds with the replacement—deleting the matches if the replacement is an empty string, as it is here. (We can specify a maximum number of matches by giving an additional integer argument at the end.)

We are careful to use nongreedy (minimal) matching to ensure that we delete one comment for each match; if we did not do this we would delete from the start of the first comment to the end of the last comment.

The re.sub() function does not accept any flags as arguments, so . means “any character except newline”, so we must look for . or \n. And we must look for these using alternation rather than a character class, since inside a character class . has its literal meaning, that is, period. An alternative would be to begin the regex with the flag embedded, for example, (?s)<!--.*?-->, or we could compile a regex object with the re.DOTALL flag, in which case the regex would simply be <!--.*?-->.

The second regex, <[Pp][^>]*?(?!</)>, matches opening paragraph tags (such as <P> or <palign=center>).It matches the opening <p (or <P), then any attributes (using nongreedy matching), and finally the closing >, providing it is not preceded by / (using a negative lookbehind assertion), since that would indicate a closing paragraph tag. The second call to the re.sub() function uses this regex to replace opening paragraph tags with two newline characters (the standard way to delimit a paragraph in a plain text file).

The third regex, <[^>]*?>, matches any tag and is used in the third re.sub() call to delete all the remaining tags.

HTML entities are a way of specifying non-ASCII characters using ASCII characters. They come in two forms: &name; where name is the name of the character—for example, &copy; for ©, and &#digits; where digits are decimal digits identifying the Unicode code point—for example, &#165; for ¥. The fourth call to re.sub() uses the regex &#(\d+);, which matches the digits form and captures the digits into capture group 1. Instead of a literal replacement text we have passed a lambda function. When a function is passed to re.sub() it calls the function once for each time it matches, passing the match object as the function’s sole argument. Inside the lambda function we retrieve the digits (as a string), convert to an integer using the built-in int() function, and then use the built-in chr() function to obtain the Unicode character for the given code point. The function’s return value (or in the case of a lambda expression, the result of the expression) is used as the replacement text.

The fifth re.sub() call uses the regex &([A-Za-z]+); to capture named entities. The standard library’s html.entities module contains dictionaries of entities, including name2codepointwhose keys are entity names and whose values are integer code points. The re.sub() function calls the local char_from_entity() function every time it has a match. The char_from_entity() function uses dict.get() with a default argument of 0xFFFD (the code point of the standard Unicode replacement character—often depicted as ? ). This ensures that a code point is always retrieved and it is used with the chr() function to return a suitable character to replace the named entity with—using the Unicode replacement character if the entity name is invalid.

The sixth re.sub() call’s regex, \n(?:[ \xA0\t]+\n)+, is used to delete lines that contain only whitespace. The character class we have used contains a space, a nonbreaking space (which &nbsp; entities are replaced with in the preceding regex), and a tab. The regex matches a newline (the one at the end of a line that precedes one or more whitespace-only lines), then at least one (and as many as possible) lines that contain only whitespace. Since the match includes the newline, from the line preceding the whitespace-only lines we must replace the match with a single newline; otherwise, we would delete not just the whitespace-only lines but also the newline of the line that preceded them.

The result of the seventh and last re.sub() call is returned to the caller. This regex, \n\n+, is used to replace sequences of two or more newlines with exactly two newlines, that is, to ensure that each paragraph is separated by just one blank line.

In the HTML example none of the replacements were directly taken from the match (although HTML entity names and numbers were used), but in some situations the replacement might need to include all or some of the matching text. For example, if we have a list of names, each of the form Forename Middlename1 ... MiddlenameN Surname, where there may be any number of middle names (including none), and we want to produce a new version of the list with each item of the form Surname, Forename Middlename1...MiddlenameN, we can easily do so using a regex:

new_names = []
for name in names:
    name = re.sub(r"(\w+(?:\s+\w+)*)\s+(\w+)", r"\2, \1", name)
    new_names.append(name)

The first part of the regex, (\w+(?:\s+\w+)*), matches the forename with the first \w+ expression and zero or more middle names with the (?:\s+\w+)* expression. The middle name expression matches zero or more occurrences of whitespace followed by a word. The second part of the regex, \s+(\w+), matches the whitespace that follows the forename (and middle names) and the surname.

If the regex looks a bit too much like line noise, we can use named capture groups to improve legibility and make it more maintainable:

name = re.sub(r"(?P<forenames>\w+(?:\s+\w+)*)"
              r"\s+(?P<surname>\w+)",
              r"\g<surname>, \g<forenames>", name)

Captured text can be referred to in a sub() or subn() function or method by using the syntax \i or \g<id> where i is the number of the capture group and id is the name or number of the capture group—so \1 is the same as \g<1>, and in this example, the same as \g<forenames>. This syntax can also be used in the string passed to a match object’s expand() method.

Why doesn’t the first part of the regex grab the entire name? After all, it is using greedy matching. In fact it will, but then the match will fail because although the middle names part can match zero or more times, the surname part must match exactly once, but the greedy middle names part has grabbed everything. Having failed, the regular expression engine will then backtrack, giving up the last “middle name” and thus allowing the surname to match.

Table 12.4 The Regular Expression Module’s Functions

Syntax

Description

re.compile(r, f) 

Returns compiled regex r with its flags set to f if specified

re.escape(s)

Returns string s with all nonalphanumeric characters backslash-escaped—therefore, the returned string has no special regex characters

re.findall(s r, s, f)

Returns all nonoverlapping matches of regex r in string (influenced by the flags f if given). If the regex has captures, each match is returned as a tuple of captures.

re.finditer(r, s, f)

Returns a match object for each nonoverlapping match of regex r in string s (influenced by the flags f if given)

re.match(r, s, f) 

Returns a match object if the regex r matches at the start of string s (influenced by the flags f if given); otherwise, returns None

re.search(r, s, f) 

Returns a match object if the regex r matches anywhere in string s (influenced by the flags f if given); otherwise, returns None

re.split(r, s, m)

Returns the list of strings that results from splitting string s on every occurrence of regex r doing up to m splits (or as many as possible if no m is given). If the regex has captures, these are included in the list between the parts they split.

re.sub(r, x, s, m) 

Returns a copy of string s with every (or up to m if given) match of regex r replaced with x—this can be a string or a function; see text

re.subn(r, x, s m) 

The same as re.sub() except that it returns a 2-tuple of the resultant string and the number of substitutions that were made

Table 12.5 The Regular Expression Module’s Flags

Flag

Meaning

re.A or re.ASCII

Makes \b, \B, \s, \S, \w, and \W assume that strings are ASCII; the default is for these character class shorthands to depend on the Unicode specification

re.I or re.IGNORECASE

Makes the regex match case-insensitively

re.M or re.MULTILINE

Makes ^ match at the start and after each newline and $ match before each newline and at the end

re.S or re.DOTALL

Makes . match every character including newlines

re.X or re.VERBOSE

Allows whitespace and comments to be included

Table 12.6 Regular Expression Object Methods

Syntax

Description

rx.findall(s start, end)

Returns all nonoverlapping matches of the regex in string s (or in the start:end slice of s). If the regex has captures, each match is returned as a tuple of captures.

rx.finditer(s start, end)

Returns a match object for each nonoverlapping match in string s (or in the start:end slice of s)

rx.flags 

The flags that were set when the regex was compiled

rx.groupindex

A dictionary whose keys are capture group names and whose values are group numbers; empty if no names are used

rx.match(s, start, end) 

Returns a match object if the regex matches at the start of string s (or at the start of the start:end slice of s); otherwise, returns None

rx.pattern 

The string from which the regex was compiled

rx.search(s, start, end)

Returns a match object if the regex matches anywhere in string s (or in the start:end slice of s); otherwise, returns None

rx.split(s, m)

Returns the list of strings that results from splitting string s on every occurrence of the regex doing up to m splits (or as many as possible if no m is given). If the regex has captures, these are included in the list between the parts they split.

rx.sub(x, s, m)

Returns a copy of string s with every (or up to m if given) match replaced with x—this can be a string or a function; see text

rx.subn(x, s m)

The same as re.sub() except that it returns a 2-tuple of the resultant string and the number of substitutions that were made

Although greedy matches match as much as possible, they stop if matching more would make the match fail.

For example, if the name is “James W. Loewen”, the regex will first match the entire name, that is, James W. Loewen. This satisfies the first part of the regex but leaves nothing for the surname part to match, and since the surname is mandatory (it has an implicit quantifier of 1), the regex has failed. Since the middle names part is quantified by *, it can match zero or more times (currently it is matching twice, “ W.” and “ Loewen”), so the regular expression engine can make it give up some of its match without causing it to fail. Therefore, the regex backtracks, giving up the last \s+\w+ (i.e., “ Loewen”), so the match becomes James W. Loewen with the match satisfying the whole regex and with the two match groups containing the correct texts.

When we use alternation (|) with two or more alternatives capturing, we don’t know which alternative matched, so we don’t know which capture group to retrieve the captured text from. We can of course iterate over all the groups to find the nonempty one, but quite often in this situation the match object’s lastindex attribute can give us the number of the group we want. We will look at one last example to illustrate this and to give us a little bit more regex practice.

Suppose we want to find out what encoding an HTML, XML, or Python file is using. We could open the file in binary mode, and read, say, the first 1000 bytes into a bytes object. We could then close the file, look for an encoding in the bytes, and reopen the file in text mode using the encoding we found or using a fallback encoding (such as UTF-8). The regex engine expects regexes to be supplied as strings, but the text the regex is applied to can be a str, bytes, or bytearray object, and when bytes or bytearray objects are used, all the functions and methods return bytes instead of strings, and the re.ASCII flag is implicitly switched on.

For HTML files the encoding is normally specified in a <meta> tag (if specified at all), for example, <meta http-equiv='Content-Type' content='text/html; charset=ISO-8859-1'/>. XML files are UTF-8 by default, but this can be overridden, for example, <?xml version="1.0"encoding="Shift_JIS"?>. Python 3 files are also UTF-8 by default, but again this can be overridden by including a line such as #encoding: latin1 or #-*-coding: latin1 -*- immediately after the shebang line.

Here is how we would find the encoding, assuming that the variable binary is a bytes object containing the first 1000 bytes of an HTML, XML, or Python file:

match = re.search(r"""(?<![-\w])                  #1
                      (?:(?:en)?coding|charset)   #2
                      (?:=(["'])?([-\w]+)(?(1)\1) #3
                      |:\s*([-\w]+))""".encode("utf8"),
                  binary, re.IGNORECASE|re.VERBOSE)
encoding = match.group(match.lastindex) if match else b"utf8"

To search a bytes object we must specify a pattern that is also a bytes object. In this case we want the convenience of using a raw string, so we use one and convert it to a bytes object as the re.search() function’s first argument.

Table 12.7 Match Object Attributes and Methods

Syntax

Description

m.end(g) 

Returns the end position of the match in the text for group g if given (or for group 0, the whole match); returns -1 if the group did not participate in the match

m.endpos 

The search’s end position (the end of the text or the end given to match() or search())

m.expand(s) 

Returns string s with capture markers (\1, \2, \g<name>, and similar) replaced by the corresponding captures

m.group(g, ...)

Returns the numbered or named capture group g; if more than one is given a tuple of corresponding capture groups is returned (the whole match is group 0)

m.groupdict(default)

Returns a dictionary of all the named capture groups with the names as keys and the captures as values; if a default is given this is the value used for capture groups that did not participate in the match

m.groups(default)

Returns a tuple of all the capture groups starting from 1; if a default is given this is the value used for capture groups that did not participate in the match

m.lastgroup 

The name of the highest numbered capturing group that matched or None if there isn’t one or if no names are used

m.lastindex 

The number of the highest capturing group that matched or None if there isn’t one

m.pos 

The start position to look from (the start of the text or the start given to match() or search())

m.re 

The regex object which produced this match object

m.span(g) 

Returns the start and end positions of the match in the text for group g if given (or for group 0, the whole match); returns (-1, -1) if the group did not participate in the match

m.start(g) 

Returns the start position of the match in the text for group g if given (or for group 0, the whole match); returns -1 if the group did not participate in the match

m.string 

The string that was passed to match() or search()

The first part of the regex itself is a lookbehind assertion that says that the match cannot be preceded by a hypen or a word character. The second part matches “encoding”, “coding”, or “charset” and could have been written as (?:encoding|coding|charset). We have made the third part span two lines to emphasise the fact that it has two alternating parts, =(["'])?([-\w]+)(?(1)\1) and :\s*([-\w]+), only one of which can match. The first of these matches an equals sign followed by one or more word or hyphen characters (optionally enclosed in matching quotes using a conditional match), and the second matches a colon and then optional whitespace followed by one or more word or hyphen characters. (Recall that a hyphen inside a character class is taken to be a literal hyphen if it is the first character; otherwise, it means a range of characters, for example, [0-9].)

We have used the re.IGNORECASE flag to avoid having to write (?:(?:[Ee][Nn])? [Cc][Oo][Dd][Ii][Nn][Gg]|[Cc][Hh][Aa][Rr][Ss][Ee][Tt]) and we have used the re.VERBOSE flag so that we can lay out the regex neatly and include comments (in this case just numbers to make the parts easy to refer to in this text).

There are three capturing match groups, all in the third part: (["'])? which captures the optional opening quote, ([-\w]+) which captures an encoding that follows an equals sign, and the second ([-\w]+) (on the following line) that captures an encoding that follows a colon. We are only interested in the encoding, so we want to retrieve either the second or third capture group, only one of which can match since they are alternatives. The lastindex attribute holds the index of the last matching capture group (either 2 or 3 when a match occurs in this example), so we retrieve whichever matched, or use a default encoding if no match was made.

We have now seen all of the most frequently used re module functionality in action, so we will conclude this section by mentioning one last function. The re.split() function (or the regex object’s split() method) can split strings based on a regex. One common requirement is to split a text on whitespace to get a list of words. This can be done using re.split(r"\s+", text) which returns a list of words (or more precisely a list of strings, each of which matches \S+). Regular expressions are very powerful and useful, and once they are learned, it is easy to see all text problems as requiring a regex solution. But sometimes using string methods is both sufficient and more appropriate. For example, we can just as easily split on whitespace by using text.split() since the str.split() method’s default behavior (or with a first argument of None) is to split on \s+.

Summary

Regular expressions offer a powerful way of searching texts for strings that match a particular pattern, and for replacing such strings with other strings which themselves can depend on what was matched.

In this chapter we saw that most characters are matched literally and are implicitly quantified by {1}. We also learned how to specify character classes—sets of characters to match—and how to negate such sets and include ranges of characters in them without having to write each character individually.

We learned how to quantify expressions to match a specific number of times or to match from a given minimum to a given maximum number of times, and how to use greedy and nongreedy matching. We also learned how to group one or more expressions together so that they can be quantified (and optionally captured) as a unit.

The chapter also showed how what is matched can be affected by using various assertions, such as positive and negative lookahead and lookbehind, and by various flags, for example, to control the interpretation of the period and whether to use case-insensitive matching.

The final section showed how to put regexes to use within the context of Python programs. In this section we learned how to use the functions provided by the re module, and the methods available from compiled regexes and from match objects. We also learned how to replace matches with literal strings, with literal strings that contain backreferences, and with the results of function calls or lambda expressions, and how to make regexes more maintainable by using named captures and comments.

Exercises

  1. In many contexts (e.g., in some web forms), users must enter a phone number, and some of these irritate users by accepting only a specific format. Write a program that reads U.S. phone numbers with the three-digit area and seven-digit local codes accepted as ten digits, or separated into blocks using hyphens or spaces, and with the area code optionally enclosed in parentheses. For example, all of these are valid: 555-555-5555, (555) 5555555, (555) 555 5555, and 5555555555. Read the phone numbers from sys.stdin and for each one echo the number in the form “(555) 555 5555” or report an error for any that are invalid.
  2. The regex to match these phone numbers is about eight lines long (in verbose mode) and is quite straightforward. A solution is provided in phone.py, which is about twenty-five lines long.

  3. Write a small program that reads an XML or HTML file specified on the command line and for each tag that has attributes, outputs the name of the tag with its attributes shown underneath. For example, here is an extract from the program’s output when given one of the Python documentation’s index.html files:
  4. html
        xmlns = http://www.w3.org/1999/xhtml
    meta
         http-equiv = Content-Type
         content = text/html; charset=utf-8
    li
         class = right
         style = margin-right: 10px

    One approach is to use two regexes, one to capture tags with their attributes and another to extract the name and value of each attribute. Attribute values might be quoted using single or double quotes (in which case they may contain whitespace and the quotes that are not used to enclose them), or they may be unquoted (in which case they cannot contain whitespace or quotes). It is probably easiest to start by creating a regex to handle quoted and unquoted values separately, and then merging the two regexes into a single regex to cover both cases. It is best to use named groups to make the regex more readable. This is not easy, especially since backreferences cannot be used inside character classes.

    A solution is provided in extract_tags.py, which is less than 35 lines long. The tag and attributes regex is just one line. The attribute name–value regex is half a dozen lines and uses alternation, conditional matching (twice, with one nested inside the other), and both greedy and nongreedy quantifiers.


Footnotes

1. A good book on regular expressions is Mastering Regular Expressions by Jeffrey E. F. Friedl, ISBN 0596528124. It does not explicitly cover Python, but Python’s re module offers very similar functionality to the Perl regular expression engine that the book covers in depth.

2. Note that backreferences cannot be used inside character classes, that is, inside [].

3. The letters used for the flags are the same as the ones used by Perl’s regex engine, which is why s is used for re.DOTALL and x is used for re.VERBOSE.



© Copyright Pearson Education. All rights reserved.

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