Python RegEx - Regular Expressions

A RegEx is a sequence of characters that form a pattern of searches.

RegEx can be used to verify whether a string has a search pattern that is specified.

Python provides an integrated re module that can be used for working with regular expressions.

Import the re module :

import re

You can start utilising regular expressions when you've imported the re module :

Example :- Check if the string starts with "The" and ends with "Spain":

import re
txt = "I am what i am"
x = re.search("^I.*am$", txt)
if x:
  print("YES! We have a match!")
else:
  print("No match")

Output :-

The re module provides a series of functions to search for a match string :

Characteristics with a specific meaning are metacharacters :

A specific sequence is a \ and a particular meaning follows one of the characters in the list below :

A set is a group of characters with unique significance inside a pair of [] square brackets.

A list of every match is returned with the findall() function.

Example 1 :- Print a list of all matches and Return a list containing every occurrence of "ai":

import re
txt = "I am what i am"
x = re.findall("am", txt)
print(x)

Output :-

The list includes the matches in the order in which they were found.

An empty list shall be returned if no matches are found :

Example 2 :- If no match was found, return an empty list :

import re
txt = "I am what i am"
#Check if "india" is in the string:
x = re.findall("india", txt)
print(x)
if (x):
  print("Yes, there is at least one match!")
else:
  print("No match")

Output :-

The function search() looks for a match string and returns a match if a match exists.

Only the first occurrence of the match can be returned if more than one match happens :

Example :- Find the first character in the string for white space :

import re
txt = "I am what i am"
x = re.search("\s", txt)
print("The first white-space character is located in position:", x.start()) 

Output :-

If there are no matches, the value None is returned :

Example 2 :- Make a search which doesn't match :

import re
txt = "I am what i am"
x = re.search("am i", txt)
print(x)

Output :-

The split() function returns a list with each match divided by the string :

Example 1 :- Split the string at every white-space character:

import re
txt = "I am what i am"
x = re.split("\s", txt)
print(x)

Output :-

The number of split can be controlled by entering the option maxsplit :

Example 2 :- Split the string at the first two white-space character :

import re
txt = "I am what i am"
x = re.split("\s", txt, 2)
print(x)

Output :-

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

Example 1 :- Replace all white-space characters with "#" :

import re
txt = "I am what i am"
x = re.sub("\s", "#", txt)
print(x)

Output :-

You can specify the count option to determine the number of replacements :

Example 2 :- Replace the first 2 occurrences of a white-space character with "#" :

import re
txt = "I am what i am"
x = re.sub("\s", "#", txt, 2)
print(x)

Output :-

Match Objects are an object with search and results information.

Note : The value of None is returned instead of the Match Object if there is no match.

Example 1 :- The search() function returns a Match object:

import re
txt = "I am what i am"
x = re.search("am", txt)
print(x)

Output :-

The object Match provides properties and methods for obtaining search information and the result :

.span() returns a tuple that contains the match start and end positions.

.string returns the string which is passed by the function.

.group() returns the part where a match exists in the string.

Example 2 :- Print the first match position (start and end).

The regular expression searches for all words that begin with "W" in the upper case :

import re
txt = "I am What i am"
x = re.search(r"\bS\w+", txt)
print(x.span())

Output :-

Example 3 :- Print the string passed into the function :

import re
txt = "I am What i am"
x = re.search(r"\bW\w+", txt)
print(x.string)

Output :-

Example 4 :- Print the string section where the match existed.

The regular expression searches for words starting with an upper case 'S' :

import re
txt = "I am What i am"
x = re.search(r"\bW\w+", txt)
print(x.group())

Output :-

Note : The None will be returned, instead of the Match Object, if no match exists.