Python RegEx

A RegEx, or Regular Expression, is a sequence of characters that forms a search pattern.

RegEx can be used to check if a string contains the specified search pattern.

RegEx Module

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

Import the re module:

import re

RegEx in Python

When you have imported the re module, you can start using regular expressions:

Example

    import
    re

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

RegEx Functions

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

Metacharacters

Metacharacters are characters with a special meaning:

Flags

You can add flags to the pattern when using regular expressions.

Special Sequences

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

Sets

A set is a set of characters inside a pair of square brackets [] with a special meaning:

The findall() Function

The findall() function returns a list containing all matches.

Example

  import re

txt = "The rain in Spain"
x = re.findall("ai",
  txt)
  print(x)

The list contains the matches in the order they are found.

If no matches are found, an empty list is returned:

Example

  import re

txt = "The rain in Spain"
x = re.findall("Portugal",
  txt)
  print(x)

The search() Function

The search() function searches the string for a match, and returns a Match object if there is a match.

If there is more than one match, only the first occurrence of the match will be returned:

Example

  import re

txt = "The rain in Spain"
x = re.search("\s",
  txt)

print("The first white-space character is located in
  position:", x.start())

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

Example

  import re

txt = "The rain in Spain"
x = re.search("Portugal",
  txt)
  print(x)

The split() Function

The split() function returns a list where the string has been split at each match:

Example

  import re

txt = "The rain in Spain"
x = re.split("\s",
  txt)
  print(x)

You can control the number of occurrences by specifying the maxsplit parameter:

Example

  import re

txt = "The rain in Spain"
x = re.split("\s",
  txt,
  1)
  print(x)

The sub() Function

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

Example

  import re

txt = "The rain in Spain"
x = re.sub("\s",
  "9", txt)
  print(x)

You can control the number of replacements by specifying the count parameter:

Example

  import re

txt = "The rain in Spain"
x = re.sub("\s",
  "9", txt, 2)
  print(x)

Match Object

A Match Object is an object containing information about the search and the result.

Note: If there is no match, the value None will be returned, instead of the Match Object.

Example

  import re

txt = "The rain in Spain"
x = re.search("ai",
  txt)
  print(x) #this will print an object

The Match object has properties and methods used to retrieve information about the search, and the result:

.span() returns a tuple containing the start-, and end positions of the match. .string returns the string passed into the function .group() returns the part of the string where there was a match

Example

import re

txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.span())

Example

import re

txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.string)

Example

import re

txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.group())

Note: If there is no match, the value None will be returned, instead of the Match Object.