Python 3 - Regular Expressions

A regular expression is a special sequence of characters that helps you match or find other strings or sets of strings, using a specialized syntax held in a pattern. Regular expressions are widely used in UNIX world.

The module re provides full support for Perl-like regular expressions in Python. The re module raises the exception re.error if an error occurs while compiling or using a regular expression.

We would cover two important functions, which would be used to handle regular expressions. Nevertheless, a small thing first: There are various characters, which would have special meaning when they are used in regular expression. To avoid any confusion while dealing with regular expressions, we would use Raw Strings as r'expression'.

Basic patterns that match single chars

Compilation flags 

Compilation flags let you modify some aspects of how regular expressions work. Flags are available in the re module under two names, a long name such as IGNORECASE and a short, one-letter form such as I.

The match Function

This function attempts to match RE pattern to string with optional flags.

Here is the syntax for this function −

re.match(pattern, string, flags = 0)

Here is the description of the parameters − 

The re.match function returns a match object on success, None on failure. We usegroup(num) or groups() function of match object to get matched expression.

Example

#!/usr/bin/python3
import re

line = "Cats are smarter than dogs"

matchObj = re.match( r'(.*) are (.*?) .*', line, re.M|re.I)

if matchObj:
   print ("matchObj.group() : ", matchObj.group())
   print ("matchObj.group(1) : ", matchObj.group(1))
   print ("matchObj.group(2) : ", matchObj.group(2))
else:
   print ("No match!!")

When the above code is executed, it produces the following result −

matchObj.group() :  Cats are smarter than dogs
matchObj.group(1) :  Cats
matchObj.group(2) :  smarter

The search Function

This function searches for first occurrence of RE pattern within string with optional flags.

Here is the syntax for this function −

re.search(pattern, string, flags = 0)

Here is the description of the parameters −

The re.search function returns a match object on success, none on failure. We use group(num) or groups() function of match object to get the matched expression.

Example

#!/usr/bin/python3
import re

line = "Cats are smarter than dogs";

searchObj = re.search( r'(.*) are (.*?) .*', line, re.M|re.I)

if searchObj:
   print ("searchObj.group() : ", searchObj.group())
   print ("searchObj.group(1) : ", searchObj.group(1))
   print ("searchObj.group(2) : ", searchObj.group(2))
else:
   print ("Nothing found!!") 

When the above code is executed, it produces the following result −

matchObj.group() :  Cats are smarter than dogs
matchObj.group(1) :  Cats
matchObj.group(2) :  smarter

Matching Versus Searching

Python offers two different primitive operations based on regular expressions: match checks for a match only at the beginning of the string, while search checks for a match anywhere in the string (this is what Perl does by default)

Example  

#!/usr/bin/python3
import re

line = "Cats are smarter than dogs";

matchObj = re.match( r'dogs', line, re.M|re.I)
if matchObj:
   print ("match --> matchObj.group() : ", matchObj.group())
else:
   print ("No match!!")

searchObj = re.search( r'dogs', line, re.M|re.I)
if searchObj:
   print ("search --> searchObj.group() : ", searchObj.group())
else:
   print ("Nothing found!!")

When the above code is executed, it produces the following result −

No match!!
search --> matchObj.group() :  dogs

Search and Replace

One of the most important re methods that use regular expressions is sub.

Syntax

re.sub(pattern, repl, string, max=0)

This method replaces all occurrences of the RE pattern in string with repl, substituting all occurrences unless max is provided. This method returns modified string.

Example

#!/usr/bin/python3
import re

phone = "2004-959-559 # This is Phone Number"

# Delete Python-style comments
num = re.sub(r'#.*$', "", phone)
print ("Phone Num : ", num)

# Remove anything other than digits
num = re.sub(r'\D', "", phone)    
print ("Phone Num : ", num) 

When the above code is executed, it produces the following result −

Phone Num : 2004-959-559 Phone Num : 2004959559  

Regular Expression Modifiers: Option Flags

Regular expression literals may include an optional modifier to control various aspects of matching. The modifiers are specified as an optional flag. You can provide multiple modifiers using exclusive OR (|), as shown previously and may be represented by one of these −

Regular Expression Patterns

Except for the control characters, (+ ? . * ^ $ ( ) [ ] { } | \), all characters match themselves. You can escape a control character by preceding it with a backslash.

The following table lists the regular expression syntax that is available in Python −

Regular Expression Examples

Literal characters

Character classes

Special Character Classes

Repetition Cases

Nongreedy repetition

This matches the smallest number of repetitions −

Grouping with Parentheses

Backreferences

This matches a previously matched group again −

Alternatives 

Anchors

This needs to specify match position. 

Special Syntax with Parentheses