Pattern-String Syntax

The pattern string representing a regular expression follows a specific syntax:

• Alphabetic and numeric characters stand for themselves. An RE whose pattern is a string of letters and digits matches the same string.

• Many alphanumeric characters acquire special meaning in a pattern when they are preceded by a backslash ().

• Punctuation works the other way around: self-matching when escaped, special meaning when unescaped.

• The backslash character is matched by a repeated backslash (i.e., pattern \).

Since RE patterns often contain backslashes, it’s best to always specify them using raw-string syntax (covered in “Strings”). Pattern elements (such as r' ', equivalent to the nonraw string literal '\t') do match the corresponding special characters (in this case, the tab character ' ', AKA chr(9)); so, you can use raw-string syntax even when you need a literal match for such special characters.

Table 9-1 lists the special elements in RE pattern syntax. The exact meanings of some pattern elements change when you use optional flags, together with the pattern string, to build the RE object. The optional flags are covered in “Optional Flags”.

Common Regular Expression Idioms

.* as a substring of a regular expression’s pattern string means “any number of repetitions (zero or more) of any character.” In other words, .* matches any substring of a target string, including the empty substring. .+ is similar, but matches only a nonempty substring. For example:

r'pre.*post'

matches a string containing a substring 'pre' followed by a later substring 'post', even if the latter is adjacent to the former (e.g., it matches both 'prepost' and 'pre23post'). On the other hand:

r'pre.+post'

matches only if 'pre' and 'post' are not adjacent (e.g., it matches 'pre23post' but does not match 'prepost'). Both patterns also match strings that continue after the 'post'. To constrain a pattern to match only strings that end with 'post', end the pattern with . For example:

r'pre.*post'

matches 'prepost', but not 'preposterous'.

All of these examples are greedy, meaning that they match the substring beginning with the first occurrence of 'pre' all the way to the last occurrence of 'post'. When you care about what part of the string you match, you may want to specify nongreedy matching, meaning to match the substring beginning with the first occurrence of 'pre' but only up to the first following occurrence of 'post'.

For example, when the string is 'preposterous and post facto', the greedy RE pattern r'pre.*post' matches the substring 'preposterous and post'; the non-greedy variant r'pre.*?post' matches just the substring 'prepost'.

Another frequently used element in RE patterns is , which matches a word boundary. To match the word 'his' only as a whole word and not its occurrences as a substring in such words as 'this' and 'history', the RE pattern is:

r'his'

with word boundaries both before and after. To match the beginning of any word starting with 'her', such as 'her' itself and 'hermetic', but not words that just contain 'her' elsewhere, such as 'ether' or 'there', use:

r'her'

with a word boundary before, but not after, the relevant string. To match the end of any word ending with 'its', such as 'its' itself and 'fits', but not words that contain 'its' elsewhere, such as 'itsy' or 'jujitsu', use:

r'its'

with a word boundary after, but not before, the relevant string. To match whole words thus constrained, rather than just their beginning or end, add a pattern element w* to match zero or more word characters. To match any full word starting with 'her', use:

r'herw*'

To match just the first three letters of any word starting with 'her', but not the word 'her' itself, use a negative word boundary B:

r'herB'

To match any full word ending with 'its', including 'its' itself, use:

r'w*its'

Sets of Characters

You denote sets of characters in a pattern by listing the characters within brackets ([]). In addition to listing characters, you can denote a range by giving the first and last characters of the range separated by a hyphen (-). The last character of the range is included in the set, differently from other Python ranges. Within a set, special characters stand for themselves, except , ], and -, which you must escape (by preceding them with a backslash) when their position is such that, if unescaped, they would form part of the set’s syntax. You can denote a class of characters within a set by escaped-letter notation, such as d or S.  in a set means a backspace character (chr(8)), not a word boundary. If the first character in the set’s pattern, right after the [, is a caret (^), the set is complemented: such a set matches any character except those that follow ^ in the set pattern notation.

A frequent use of character sets is to match a word using a definition of which characters can make up a word that differs from w’s default (letters and digits). To match a word of one or more characters, each of which can be a letter, an apostrophe, or a hyphen, but not a digit (e.g., "Finnegan-O'Hara"), use:

r"[a-zA-Z'-]+"

Alternatives

A vertical bar (|) in a regular expression pattern, used to specify alternatives, has low syntactic precedence. Unless parentheses change the grouping, | applies to the whole pattern on either side, up to the start or end of the pattern, or to another |. A pattern can be made up of any number of subpatterns joined by |. To match such an RE, the first subpattern is tried, and if it matches, the others are skipped. If the first subpattern doesn’t match, the second subpattern is tried, and so on. | is neither greedy nor nongreedy: it just doesn’t take the length of the match into account.

Given a list L of words, an RE pattern that matches any one of the words is:

'|'.join(r'{}'.format(word) for word in L)

'|'.join(re.escape(s) for s in L)

Groups

A regular expression can contain any number of groups, from none to 99 (or even more, but only the first 99 groups are fully supported). Parentheses in a pattern string indicate a group. Element (?P<id>...) also indicates a group, and gives the group a name, id, that can be any Python identifier. All groups, named and unnamed, are numbered from left to right, 1 to 99; “group 0” means the whole RE.

For any match of the RE with a string, each group matches a substring (possibly an empty one). When the RE uses |, some groups may not match any substring, although the RE as a whole does match the string. When a group doesn’t match any substring, we say that the group does not participate in the match. An empty string ('') is used as the matching substring for any group that does not participate in a match, except where otherwise indicated later in this chapter. For example:

r'(.+)1+'

matches a string made up of two or more repetitions of any nonempty substring. The (.+) part of the pattern matches any nonempty substring (any character, one or more times) and defines a group, thanks to the parentheses. The 1+ part of the pattern matches one or more repetitions of the group, and  anchors the match to the end of the string.

Optional Flags

A regular expression pattern element with one or more of the letters iLmsux between (? and ) lets you set RE options within the pattern, rather than by the flags argument to the compile function of the re module. Options apply to the whole RE, no matter where the options element occurs in the pattern.

Using the explicit flags argument is more readable than placing an options element within the pattern. The flags argument to the function compile is a coded integer built by bitwise ORing (with Python’s bitwise OR operator, |) one or more of the following attributes of the module re. Each attribute has both a short name (one uppercase letter), for convenience, and a long name (an uppercase multiletter identifier), which is more readable and thus normally preferable:

I or IGNORECASE

Makes matching case-insensitive

L or LOCALE

Causes w, W, , and B matches to depend on what the current locale deems alphanumeric; deprecated in v3

M or MULTILINE

Makes the special characters ^ and $ match at the start and end of each line (i.e., right after/before a newline), as well as at the start and end of the whole string (A and  always match only the start and end of the whole string)

S or DOTALL

Causes the special character . to match any character, including a newline

U or UNICODE

Makes w, W, , and B matches depend on what Unicode deems alphanumeric; deprecated in v3

X or VERBOSE

Causes whitespace in the pattern to be ignored, except when escaped or in a character set, and makes a # character in the pattern begin a comment that lasts until the end of the line

For example, here are three ways to define equivalent REs with function compile, covered in Table 9-3. Each of these REs matches the word “hello” in any mix of upper- and lowercase letters:

import re
r1 = re.compile(r'(?i)hello')
r2 = re.compile(r'hello', re.I)
r3 = re.compile(r'hello', re.IGNORECASE)

The third approach is clearly the most readable, and thus the most maintainable, even though it is slightly more verbose. The raw-string form is not necessary here, since the patterns do not include backslashes; however, using raw strings is innocuous, and we recommend you always do that for RE patterns, to improve clarity and readability.

Option re.VERBOSE (or re.X) lets you make patterns more readable and understandable by appropriate use of whitespace and comments. Complicated and verbose RE patterns are generally best represented by strings that take up more than one line, and therefore you normally want to use the triple-quoted raw-string format for such pattern strings. For example, to match old-style Python v2 int literals:

repat_num1 = r'(0[0-7]*|0x[da-fA-F]+|[1-9]d*)L?'
repat_num2 = r'''(?x)            # pattern matching int literals
              (0 [0-7]*        | # octal: leading 0, 0+ octal digits
               0x [da-fA-F]+  | # hex: 0x, then 1+ hex digits
               [1-9] d*       ) # decimal: leading non-0, 0+ digits
               L?              # optional trailing L, end of string
              '''

The two patterns defined in this example are equivalent, but the second one is made more readable and understandable by the comments and the free use of whitespace to visually group portions of the pattern in logical ways.

Match Versus Search

So far, we’ve been using regular expressions to match strings. For example, the RE with pattern r'box' matches strings such as 'box' and 'boxes', but not 'inbox'. In other words, an RE match is implicitly anchored at the start of the target string, as if the RE’s pattern started with A.

Often, you’re interested in locating possible matches for an RE anywhere in the string, without anchoring (e.g., find the r'box' match inside such strings as 'inbox', as well as in 'box' and 'boxes'). In this case, the Python term for the operation is a search, as opposed to a match. For such searches, use the search method of an RE object; the match method deals with matching only from the start. For example:

import re
r1 = re.compile(r'box')
if r1.match('inbox'):
    print('match succeeds')
else:
    print('match fails')         # prints: match fails
if r1.search('inbox'):
    print('search succeeds')     # prints: search succeeds
else:
    print('search fails')

Anchoring at String Start and End

The pattern elements ensuring that a regular expression search (or match) is anchored at string start and string end are A and , respectively. More traditionally, elements ^ for start and $ for end are also used in similar roles. ^ is the same as A, and $ is the same as , for RE objects that are not multiline (i.e., that do not contain pattern element (?m) and are not compiled with the flag re.M or re.MULTILINE). For a multiline RE, however, ^ anchors at the start of any line (i.e., either at the start of the whole string or at any position right after a newline character ). Similarly, with a multiline RE, $ anchors at the end of any line (i.e., either at the end of the whole string or at any position right before a ). On the other hand, A and  anchor at the start and end of the string whether the RE object is multiline or not. For example, here’s a way to check whether a file has any lines that end with digits:

import re
digatend = re.compile(r'd$', re.MULTILINE)
with open('afile.txt') as f:
    if digatend.search(f.read()): 
        print('some lines end with digits')
    else:
        print('no line ends with digits')

A pattern of r'd ' is almost equivalent, but in that case the search fails if the very last character of the file is a digit not followed by an end-of-line character. With the preceding example, the search succeeds if a digit is at the very end of the file’s contents, as well as in the more usual case where a digit is followed by an end-of-line character.

Regular Expression Objects

A regular expression object r has the following read-only attributes that detail how r was built (by the function compile of the module re, covered in Table 9-3):

flags

The flags argument passed to compile, or 0 when flags is omitted (in v2; in v3, the default when flags is omitted is re.UNICODE)

groupindex

A dictionary whose keys are group names as defined by elements (?P<id>...); the corresponding values are the named groups’ numbers

pattern

The pattern string from which r is compiled

These attributes make it easy to get back from a compiled RE object to its pattern string and flags, so you never have to store those separately.

An RE object r also supplies methods to locate matches for r within a string, as well as to perform substitutions on such matches (Table 9-2). Matches are generally represented by special objects, covered in “Match Objects”.

import re
reword = re.compile(r'w+')
with open('afile.txt') as f:
    for aword in reword.findall(f.read()):
        print(aword)

reword = re.compile('(w+)s')

import re
first_last = 
  re.compile(r'^W*(w+).*(w+)W*$',re.MULTILINE)
with open('afile.txt') as f:
    for first, last in first_last.findall(f.read()):
        print(first, last)

import re
digs = re.compile(r'd')
with open('afile.txt') as f:
    for line in f:
        if digs.match(line):
            print(line, end='')

import re
digs = re.compile(r'd')
with open('afile.txt') as f:
    for line in f:
        if digs.search(line):
            print(line, end='')

import re
rehello = re.compile(r'hello', re.IGNORECASE)
astring = ''.join(rehello.split(astring))

import re
re_col_ws_dig = re.compile(r'(:)s+(d)')
astring = ''.join(re_col_ws_dig.split(astring))

astring = ''.join(rehello.split(astring, 1))

import re
rehello = re.compile(r'hello', re.IGNORECASE)
astring = rehello.sub('', astring, 1)

import re
h_word = re.compile(r'hw*o', re.IGNORECASE)
def up(mo): return mo.group(0).upper()
astring = h_word.sub(up, astring)

import re
grouped_word = re.compile('(w+)')
astring = grouped_word.sub(r'{1}', astring)

import re
rehello = re.compile(r'hello', re.IGNORECASE)
_, count = rehello.subn('', astring)
print('Found', count, 'occurrences of "hello"')

Match Objects

Match objects are created and returned by the methods match and search of a regular expression object, and are the items of the iterator returned by the method finditer. They are also implicitly created by the methods sub and subn when the argument repl is callable, since in that case the appropriate match object is passed as the argument on each call to repl. A match object m supplies the following read-only attributes that detail how m was created:

pos

The start argument that was passed to search or match (i.e., the index into s where the search for a match began)

endpos

The end argument that was passed to search or match (i.e., the index into s before which the matching substring of s had to end)

lastgroup

The name of the last-matched group (None if the last-matched group has no name, or if no group participated in the match)

lastindex

The integer index (1 and up) of the last-matched group (None if no group participated in the match)

re

The RE object r whose method created m

string

The string s passed to finditer, match, search, sub, or subn

A match object m also supplies several methods, detailed in Table 9-3.

Functions of the re Module

The re module supplies the attributes listed in “Optional Flags”. It also provides one function for each method of a regular expression object (findall, finditer, match, search, split, sub, and subn), each with an additional first argument, a pattern string that the function implicitly compiles into an RE object. It’s usually better to compile pattern strings into RE objects explicitly and call the RE object’s methods, but sometimes, for a one-off use of an RE pattern, calling functions of the module re can be slightly handier. For example, to count the number of occurrences of 'hello' in any mix of cases, one concise, function-based way is:

import re
_, count = re.subn(r'(?i)hello', '', astring)
print('Found', count, 'occurrences of "hello"')

In such cases, RE options (here, case insensitivity) must be encoded as RE pattern elements (here (?i)): the functions of the re module do not accept a flags argument. The re module internally caches RE objects it creates from the patterns passed to functions; to purge the cache and reclaim some memory, call re.purge().

The re module also supplies error, the class of exceptions raised upon errors (generally, errors in the syntax of a pattern string), and two more functions: