RegEx in Python

A Regular Expression (RegEx) is a sequence of characters that forms a search pattern. It can be used to check if a string contains the specified search pattern.

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The re Module

Python has a built-in package called re, which can be used to work with Regular Expressions.

import re

txt = "The rain in Spain"
x = re.search("^The.*Spain$", txt)

if x:
  print("YES! We have a match!")
else:
  print("No match")

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RegEx Functions

The re module offers a set of functions that allows us to search a string for a match:

• findall(): Returns a list containing all matches.
• search(): Returns a Match object if there is a match anywhere in the string.
• split(): Returns a list where the string has been split at each match.
• sub(): Replaces one or many matches with a string.

Example: findall()

import re

txt = "The rain in Spain"
x = re.findall("ai", txt)
print(x) # Output: ['ai', 'ai']

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Metacharacters

Metacharacters are characters with a special meaning:

Character	Description	Example
[]	A set of characters	"[a-m]"
\	Signals a special sequence	"\d"
.	Any character (except newline character)	"he..o"
^	Starts with	"^hello"
$	Ends with	"planet$"
*	Zero or more occurrences	"he.*o"
+	One or more occurrences	"he.+o"
?	Zero or one occurrences	"he.?o"
{}	Exactly the specified number of occurrences	"he.{2}o"
|	Either or	"falls|stays"

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Special Sequences

A special sequence is a \ followed by one of the characters in the list below, and has a special meaning:

Character	Description	Example
\d	Returns a match where the string contains digits (numbers from 0-9)	"\d"
\D	Returns a match where the string DOES NOT contain digits	"\D"
\s	Returns a match where the string contains a white space character	"\s"
\S	Returns a match where the string DOES NOT contain a white space character	"\S"
\w	Returns a match where the string contains any word characters	"\w"
\W	Returns a match where the string DOES NOT contain any word characters	"\W"

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The sub() Function

The sub() function replaces the matches with the text of your choice:

import re

txt = "The rain in Spain"
# Replace every white-space character with the number 9:
x = re.sub("\s", "9", txt)
print(x) # Output: The9rain9in9Spain