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Python Flags to Tune the Behavior of Regular Expressions
  • Last Updated : 01 Oct, 2020

Python offers some flags to modify the behavior of regular expression engines. Let’s discuss them below: 

  1. Case Insensitivity
  2. Dot Matching Newline
  3. Multiline Mode
  4. Verbose Mode
  5. Debug Mode

Case Insensitivity

The re.IGNORECASE allows the regular expression to become case-insensitive. Here, the match is returned based on the case of the provided string, not the string in the regular expression.

Python3




import re
  
  
match = re.search(r'geeksforgeeks', 'GeeksforGeeks',re.IGNORECASE)
print(match)
Output
<_sre.SRE_Match object; span=(0, 13), match='GeeksforGeeks'>

Dot Matching Newline

By using re.DOTALL flag, you can modify the behavior of dot (.) character to match the newline character apart from other characters. Before using the DOTALL flag, let’s look into how regular engine responds to the newline character.

Python3




import re
  
  
match = re.search(r'.+', 'Hello,\nGeeks')
print(match)
Output



<_sre.SRE_Match object; span=(0, 6), match='Hello,'>

Here, the regular expression matches one or more characters (‘. +’). At the time when the engine reaches the newline character, it stops, because the dot character doesn’t match the line breaks. Let’s look into the code that makes use of the DOTALL flag.

Python3




import re
  
  
match = re.search(r'.+','Hello,\nGeeks', re.DOTALL)
print(match)

Output:

<_sre.SRE_Match object; span=(0, 12), match=’Hello,\nGeeks’>

Multiline mode

With the Multiline flag, you can match against the beginning and the end of any line within the string. If we look into the ^ character, it will only match against the beginning of a string. So, even if there is a matching character after the newline character, It returns none. Let’s look into the below code.

Python3




import re
  
  
match = re.search(r'^Geeks', 'Hello,\nGeeks')
print(match)
Output
None

Using the Multiline flag, you can overcome the above issue. It can match against the beginning and end of any line in the string. Let’s match against the beginning of a string.

Python3




import re
  
  
match = re.search(r'^Geeks', 'Hello,\nGeeks', re.MULTILINE)
print(match)
Output
<_sre.SRE_Match object; span=(7, 12), match='Geeks'>

Verbose Mode

It allows representing a regular expression in a more readable way. Let’s look at the below code.



Python3




import re
  
  
match = re.search(r"""(?P<first_two>[\d]{2})  # The first two digits
          -                           # A literal python 
          (?P<last_three>[\d]{3})     # The last three digit
          """, '25-542', re.VERBOSE)
print(match)
Output
<_sre.SRE_Match object; span=(0, 6), match='25-542'>

The Verbose flag treats # character as a comment character and also ignores all the whitespace characters including the line break.

Debug Mode

The re.DEBUG flag provides debugging information while compiling a regular expression. Let’s have a look at the below code.

Python3




import re
  
  
match = re.search(r'(?P<first_two>[\d]{2})-(?P<last_three>[\d]{3})',\
                  '25-542', re.DEBUG)
print(match)
Output
SUBPATTERN 1 0 0
  MAX_REPEAT 2 2
    IN
      CATEGORY CATEGORY_DIGIT
LITERAL 45
SUBPATTERN 2 0 0
  MAX_REPEAT 3 3
    IN
      CATEGORY CATEGORY_DIGIT
<_sre.SRE_Match object; span=(0, 6), match='25-542'>

Here, you have seen different types of flags that can slightly change the behavior of a regular expression engine. You can also use multiple flags at the same time by using a bitwise OR (|) operator. 

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