Defining a regular expression is to provide a sequence of characters, the pattern, that will match sequences of characters in a target.

Here are several places to look for help:

• Python Library Reference: 4.2.1 Regular Expression Syntax
• Regular Expression HOWTO

The patterns or regular expressions can be defined as follows:

• Literal characters must match exactly. For example, "a" matches "a".
• Concatenated patterns match concatenated targets. For example, "ab" ("a" followed by "b") matches "ab".
• Alternate patterns, separated by a vertical bar, match either of the alternative patterns. For example, "(aaa)|(bbb)" will match either "aaa" or "bbb".
• Repeating and optional items:
  • "abc*" matches "ab" followed by zero or more occurances of "c", for example, "ab", "abc", "abcc", etc.
  • "abc+" matches "ab" followed by one or more occurances of "c", for example, "abc", "abcc", etc, but not "ab".
  • "abc?" matches "ab" followed by zero or one occurances of "c", for example, "ab" or "abc".
• Sets of characters -- Characters and sequences of characters in square brackets form a set; a set matches any character in the set or range. For example, "[abc]" matches "a" or "b" or "c". And, for example, "[_a-z0-9]" matches an underscore or any lower-case letter or any digit.
• Groups -- Parentheses indicate a group with a pattern. For example, "ab(cd)*ef" is a pattern that matches "ab" followed by any number of occurances of "cd" followed by "ef", for example, "abef", "abcdef", "abcdcdef", etc.
• There are special names for some sets of characters, for example "\d" (any digit), "\w" (any alphanumeric character), "\W" (any non-alphanumeric character), etc. See Python Library Reference: 4.2.1 Regular Expression Syntax for more.

Because of the use of backslashes in patterns, you are usually better off defining regular expressions with raw strings, e.g. r"abc".

Compiling regular expressions

When a regular expression is to be used more than once, you should consider compiling it. For example:

import sys, re



pat = re.compile('aa[bc]*dd')



while 1:

    line = raw_input('Enter a line ("q" to quit):')

    if line == 'q':

        break

    if pat.search(line):

        print 'matched:', line

    else:

        print 'no match:', line

Comments:

• We import module re in order to use regular expresions.
• "re.compile()" compiles a regular expression so that we can reuse the compiled regular expression without compiling it repeatedly.

Using regular expressions

Use match to match at the beginning of a string (or not at all).

Use search to search a string and match the first string from the left.

Here are some examples:

>>> import re

>>> pat = re.compile('aa[0-9]*bb')

>>> x = pat.match('aa1234bbccddee')

>>> x

<_sre.SRE_Match object at 0x401e9608>

>>> x = pat.match('xxxxaa1234bbccddee')

>>> x

>>> type(x)

<type 'NoneType'>

>>> x = pat.search('xxxxaa1234bbccddee')

>>> x

<_sre.SRE_Match object at 0x401e9608>

Notes:

• When a match or search is successful, it returns a match object. When it fails, it returns None.
• You can also call the corresponding functions match and search in the re module, e.g.:

  >>> x = re.search(pat, 'xxxxaa1234bbccddee')
  
  >>> x
  
  <_sre.SRE_Match object at 0x401e9560>
  
  

Using match objects to extract a value

Match objects enable you to extract matched sub-strings after performing a match. A match object is returned by successful match.

Here is an example:

import sys, re



pat = re.compile('aa([0-9]*)bb')



while 1:

    line = raw_input('Enter a line ("q" to quit):')

    if line == 'q':

        break

    mo = pat.search(line)

    if mo:

        value = mo.group(1)

        print 'value: %s' % value

    else:

        print 'no match'

Explanation:

• In the regular expression, put parentheses around the portion of the regular expression that will match what you want to extract. Each pair of parentheses marks off a group.
• After the search, check to determine if there was a successful match by checking for a matching object. "pat.search(line)" returns None if the search fails.
• If you specify more than one group in your regular expression (more that one pair of parentheses), then you can use "value = mo.group(N)" to extract the value matched by the Nth group from the matching object. "value = mo.group(1)" returns the first extracted value; "value = mo.group(2)" returns the second; etc. An argument of 0 returns the string matched by the entire regular expression.

In addition, you can:

• Use "values = mo.groups()" to get a tuple containing the strings matched by all groups.
• Use "mo.expand()" to interpolate the group values into a string. For example, "mo.expand(r'value1: \1 value2: \2')"inserts the values of the first and second group into a string. If the first group matched "aaa" and the second matched "bbb", then this example would produce "value1: aaa value2: bbb".

Extracting multiple items

You can extract multiple items with a single search. Here is an example:

import sys, re



pat = re.compile('aa([0-9]*)bb([0-9]*)cc')



while 1:

    line = raw_input('Enter a line ("q" to quit):')

    if line == 'q':

        break

    mo = pat.search(line)

    if mo:

        value1, value2 = mo.group(1, 2)

        print 'value1: %s  value2: %s' % (value1, value2)

    else:

        print 'no match'

Comments:

• Use multiple parenthesized substrings in the regular expression to indicate the portions (groups) to be extracted.
• "mo.group(1, 2)" returns the values of the first and second group in the string matched.
• We could also have used "mo.groups()" to obtain a tuple that contains both values.
• Yet another alternative would have been to use the following; "print mo.expand(r'value1: \1 value2: \2')".

Replacing multiple items

You can locate sub-strings (slices) of a match and replace them. Here is an example:

import sys, re



pat = re.compile('aa([0-9]*)bb([0-9]*)cc')



while 1:

    line = raw_input('Enter a line ("q" to quit): ')

    if line == 'q':

        break

    mo = pat.search(line)

    if mo:

        value1, value2 = mo.group(1, 2)

        start1 = mo.start(1)

        end1 = mo.end(1)

        start2 = mo.start(2)

        end2 = mo.end(2)

        print 'value1: %s  start1: %d  end1: %d' % (value1, start1, end1)

        print 'value2: %s  start2: %d  end2: %d' % (value2, start2, end2)

        repl1 = raw_input('Enter replacement #1: ')

        repl2 = raw_input('Enter replacement #2: ')

        newline = line[:start1] + repl1 + line[end1:start2] + repl2 + line[end2:]

        print 'newline: %s' % newline

    else:

        print 'no match'

Explanation:

• Alternatively, use "mo.span(1)" instead of "mo.start(1)" and "mo.end(1)" in order to get the start and end of a sub-match in a single operation. "mo.span(1)"returns a tuple: (start, end).
• Put together a new string with string concatenation from pieces of the original string and replacement values. You can use string slices to get the sub-strings of the original string. In our case, the following gets the start of the string, adds the first replacement, adds the middle of the original string, adds the second replacement, and finally, adds the last part of the original string:

  newline = line[:start1] + repl1 + line[end1:start2] + repl2 + line[end2:]
  
  

You can also use the sub function or method to do substitutions. Here is an example:

import sys, re



pat = re.compile('[0-9]+')



print 'Replacing decimal digits.'

while 1:

    target = raw_input('Enter a target line ("q" to quit): ')

    if target == 'q':

        break

    repl = raw_input('Enter a replacement: ')

    result = pat.sub(repl, target)

    print 'result: %s' % result

And, finally, you can define a function to be used to insert calculated replacements. Here is an example:

import sys, re, string



pat = re.compile('[a-m]+')



def replacer(mo):

    return string.upper(mo.group(0))



print 'Upper-casing a-m.'

while 1:

    target = raw_input('Enter a target line ("q" to quit): ')

    if target == 'q':

        break

    result = pat.sub(replacer, target)

    print 'result: %s' % result

Notes:

• If the replacement argument to sub is a function, that function must take one argument, a match object, and must return the modified (or replacement) value. The matched sub-string will be replaced by the value returned by this function.
• In our case, the function replacer converts the matched value to upper case.

This is also a convenient use for a lambda instead of a named function, for example:

import sys, re, string



pat = re.compile('[a-m]+')



print 'Upper-casing a-m.'

while 1:

    target = raw_input('Enter a target line ("q" to quit): ')

    if target == 'q':

        break

    result = pat.sub(

        lambda mo: string.upper(mo.group(0)),

        target)

    print 'result: %s' % result