29. RegEx Or Regular Expressio

By | October 6, 2021

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

Search the string to see if it starts with “The” and ends with “Spain”:
import re

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

Output:
YES! We have a match!

RegEx Functions

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

FunctionDescription
findallReturns a list containing all matches
searchReturns a Match object if there is a match anywhere in the string
splitReturns a list where the string has been split at each match
subReplaces one or many matches with a string

The findall() Function

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

Example

Print a list of all matches:
import re
#Return a list containing every occurrence of “ai”:
txt = “The rain in Spain”
x = re.findall(“ai”, txt)
print(x)

Output:
[‘ai’, ‘ai’]

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

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

Example

Return an empty list if no match was found:
import re
txt = “The rain in Spain”
#Check if “Portugal” is in the string:
x = re.findall(“Portugal”, txt)
print(x)

if (x):
print(“Yes, there is at least one match!”)
else:
print(“No match”)

Output:
[]
No match

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

Search for the first white-space character in the string:
import re
txt = “The rain in Spain”
x = re.search(“\s”, txt)

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

Output:
The first white-space character is located in position: 3

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

Example

Make a search that returns no match:
import re
txt = “The rain in Spain”
x = re.search(“Portugal”, txt)
print(x)

Output:
None

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)

Output:
[‘The’, ‘rain’, ‘in’, ‘Spain’]

You 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)

Output:
[‘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)

Output:
The9rain9in9Spain

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

Example

Replace the first 2 occurrences:
import re
txt = “The rain in Spain”
x = re.sub(“\s”, “9”, txt, 2)
print(x)

Output:
The9rain9in Spain

Metacharacters

Example

import re
txt = “The rain in Spain”
#Find all lower case characters alphabetically between “a” and “m”:
x = re.findall(“[a-m]”, txt)
print(x)

Output:
[‘h’, ‘e’, ‘a’, ‘i’, ‘i’, ‘a’, ‘i’]

Metacharacters are characters with a special meaning:

CharacterDescriptionExample
[]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{2}”
|Either or“falls|stays”
()Capture and group 

Special Sequences

Example

import re
txt = “The rain in Spain”
#Check if the string starts with “The”:
x = re.findall(“\AThe”, txt)
print(x)
if x:
print(“Yes, there is a match!”)
else:
print(“No match”)

Output:
[‘The’]
Yes, there is a match!

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

CharacterDescriptionExample
\AReturns a match if the specified characters are at the beginning of the string“\AThe”
\bReturns 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”
\BReturns 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”
\dReturns a match where the string contains digits (numbers from 0-9)“\d”
\DReturns a match where the string DOES NOT contain digits“\D”
\sReturns a match where the string contains a white space character“\s”
\SReturns a match where the string DOES NOT contain a white space character“\S”
\wReturns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character)“\w”
\WReturns a match where the string DOES NOT contain any word characters“\W”
\ZReturns a match if the specified characters are at the end of the string“Spain\Z”

Sets

Example

import re
txt = “The rain in Spain”
#Check if the string has any a, r, or n characters:
x = re.findall(“[arn]”, txt)
print(x)
if x:
print(“Yes, there is at least one match!”)
else:
print(“No match”)

Output:
[‘r’, ‘a’, ‘n’, ‘n’, ‘a’, ‘n’]
Yes, there is at least one match!

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

SetDescription
[arn]Returns a match where one of the specified characters (ar, 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 ar, and n
[0123]Returns a match where any of the specified digits (012, 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, +*.|()$,{} has no special meaning, so [+] means: return a match for any + character in the string

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

Do a search that will return a Match Object:
import re
txt = “The rain in Spain”
x = re.search(“ai”, txt)
print(x) #this will print an object

Output:
<_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())

Output:
(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)

Output:
The rain in Spain

Example

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

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.group())

Output:
Spain

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

Leave a Reply

Your email address will not be published. Required fields are marked *