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)
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 Example
[] 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
A special sequence is a \ followed by one of the characters in the list below, and has a special meaning:
Character Description Example
\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, +, *, ., |, (), $,{} has no special meaning, so [+] means: return a match for any + character in the string
The findall() Function
The findall() function returns a list containing all matches.
Example
Print a list of all matches:
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
Return an empty list if no match was found:
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
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())
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)
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)
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)
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)
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)
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
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())
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)
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())
Note: If there is no match, the value None will be returned, instead of the Match Object.
