Regular Expressions

Regular Expressions

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.

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

RegEx Functions

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

Function	Description
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

Metacharacters

Metacharacters are characters with a special meaning:

Character	Description	Usage
[]	A set of characters	"[a-m]"
\	Signals a special sequence (can also be used to escape special characters)	"\d"
.	Any character (except newline character)	"he..o"
^	Starts with	"^hello"
$	Ends with	"world$"
*	Zero or more occurrences	"aix*"
+	One or more occurrences	"aix+"
{}	Exactly the specified number of occurrences	"al
|	Either or	"falls

Special Sequences

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

Character	Description	Usage
\A	Returns a match if the specified characters are at the beginning of the string	"\AThe"
\b	Returns a match where the specified characters are at the beginning or at the end of a word (the "r" in the beginning is making sure that the string is being treated as a "raw string")	r"\bain" r"ain\b"
\B	Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word (the "r" in the beginning is making sure that the string is being treated as a "raw string")	r"\Bain" r"ain\B"
\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 (characters from a to Z, digits from 0-9, and the underscore _ character)	"\w"
\W	Returns a match where the string DOES NOT contain any word characters	"\W"
\Z	Returns a match if the specified characters are at the end of the string	"Spain\Z"

Sets

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

Set	Description
[arn]	Returns a match where one of the specified characters (a, r, or n) are present
[a-n]	Returns a match for any lower case character, alphabetically between a and n
[^arn]	Returns a match for any character EXCEPT a, r, and n
[0123]	Returns a match where any of the specified digits (0, 1, 2, or 3) are present
[0-9]	Returns a match for any digit between 0 and 9
[0-5][0-9]	Returns a match for any two-digit numbers from 00 and 59
[a-zA-Z]	Returns a match for any character alphabetically between a and z, lower case OR upper case
[+]	In sets, +, *, .,

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)
>>> ['ai', 'ai']

The list contains the matches in the order they are found. If no matches are found, an empty list is returned.

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.

Search for the white space character

import re

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

print("The first white-space character is located in position:", x.start())
>>> The first white-space character is located in position: 3

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

Match Object

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

Search returns a match object

import re

txt = "The rain in Spain"
x = re.search("ai", txt)
print(x) 
>>> <_sre.SRE_Match object; span=(5, 7), match='ai'>

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

Print the position (start- and end-position) of the first match occurrence. The regular expression looks for any words that starts with an upper case "S".

import re

txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.span())
>>> (12, 17)

Example

Print the string passed into the function.

import re

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

Example

Print the part of the string where there was a match.

import re

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

The split() Function

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

Example

Split at each white-space character.

import re

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

We can control the number of occurrences by specifying the maxsplit parameter.

Example

Split the string only at the first occurrence.

import re

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

The sub() Function

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

Example

Replace every white-space character with the number 9.

import re

txt = "The rain in Spain"
x = re.sub("\s", "9", txt)
print(x)
>>> The9rain9in9Spain

We can control the number of replacements by specifying the count parameter.

Example

Replace the first 2 occurrences only.

import re

txt = "The rain in Spain"
x = re.sub("\s", "9", txt, 2)
print(x)
>>> The9rain9in Spain