Regular Expressions in Python

Regular Expressions in Python are very important in programming, Python "re" module provides regular expression support.

The re.search() method takes a regular expression pattern and a string and searches for that pattern within the string. If the search is successful, search() returns a match object or None otherwise. Therefore, the search is usually immediately followed by an if-statement to test if the search succeeded, as shown in the following example which searches for the pattern 'word:' followed by a 3 letter word (details below):

str = 'an example word:cat!!'
match = re.search(r'word:\w\w\w', str)
# If-statement after search() tests if it succeeded
  if match:                      
    print 'found', match.group() ## 'found word:cat'
  else:

    print 'did not find'

The code match = re.search(pat, str) stores the search result in a variable named "match". Then the if-statement tests the match -- if true the search succeeded and match.group() is the matching text (e.g. 'word:cat'). Otherwise if the match is false (None to be more specific), then the search did not succeed, and there is no matching text.


The 'r' at the start of the pattern string designates a python "raw" string which passes through backslashes without change which is very handy for regular expressions (Java needs this feature badly!). I recommend that you always write pattern strings with the 'r' just as a habit.

str = 'an example word:cat!!'
match = re.search(r'word:\w\w\w', str)
# If-statement after search() tests if it succeeded
  if match:                      
    print 'found', match.group() ## 'found word:cat'
  else:
    print 'did not find'

str = 'an example word:cat!!'
match = re.search(r'word:\w\w\w', str)
# If-statement after search() tests if it succeeded
  if match:                      
    print 'found', match.group() ## 'found word:cat'
  else:
    print 'did not find'

str = 'an example word:cat!!'
match = re.search(r'word:\w\w\w', str)
# If-statement after search() tests if it succeeded
  if match:                      
    print 'found', match.group() ## 'found word:cat'
  else:
    print 'did not find'
str = 'an example word:cat!!'
match = re.search(r'word:\w\w\w', str)
# If-statement after search() tests if it succeeded
  if match:                      
    print 'found', match.group() ## 'found word:cat'
  else:
    print 'did not find'