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A Regular Expression or RegEx is a special sequence of characters that uses a search pattern to find a string or set of strings.

It can detect the presence or absence of a text by matching it with a particular pattern and also can split a pattern into one or more sub-patterns.

Regex Module in Python

Python has a built-in module named "re" that is used for regular expressions in Python. We can import this module by using the import statement.

Example: Importing re module in Python

# importing re module
import re

How to Use RegEx in Python?

You can use RegEx in Python after importing re module.

Example:

This Python code uses regular expressions to search for the word "portal" in the given string and then prints the start and end indices of the matched word within the string.

import re

s = 'GeeksforGeeks: A computer science portal for geeks'

match = re.search(r'portal', s)

print('Start Index:', match.start())
print('End Index:', match.end())

Start Index: 34
End Index: 40

Note: Here r character (r’portal’) stands for raw, not regex. The raw string is slightly different from a regular string, it won’t interpret the \ character as an escape character. This is because the regular expression engine uses \ character for its own escaping purpose.

Before starting with the Python regex module let's see how to actually write regex using metacharacters or special sequences. 

RegEx Functions

The re module in Python provides various functions that help search, match, and manipulate strings using regular expressions.

Below are the main functions available in the re module:

Let's see the working of these RegEx functions with definition and examples:

1. re.findall()

Returns all non-overlapping matches of a pattern in the string as a list. It scans the string from left to right.

This code uses the regular expression \d+ to find all sequences of one or more digits in the given string.

import re
string = """Hello my Number is 123456789 and
            my friend's number is 987654321"""
regex = '\d+'

match = re.findall(regex, string)
print(match)

['123456789', '987654321']

2. re.compile()

Compiles a regex into a pattern object, which can be reused for matching or substitutions.

Example 1: This pattern [a-e] matches all lowercase letters between 'a' and 'e', in the input string "Aye, said Mr. Gibenson Stark". The output should be ['e', 'a', 'd', 'b', 'e'], which are the matching characters.

import re
p = re.compile('[a-e]')

print(p.findall("Aye, said Mr. Gibenson Stark"))

['e', 'a', 'd', 'b', 'e', 'a']

Explanation:

• First occurrence is 'e' in "Aye" and not 'A', as it is Case Sensitive.
• Next Occurrence is 'a' in "said", then 'd' in "said", followed by 'b' and 'e' in "Gibenson", the Last 'a' matches with "Stark".
• Metacharacter backslash '\' has a very important role as it signals various sequences. If the backslash is to be used without its special meaning as metacharacter, use'\\'

Example 2: The code uses regular expressions to find and list all single digits and sequences of digits in the given input strings. It finds single digits with \d and sequences of digits with \d+.

import re
p = re.compile('\d')
print(p.findall("I went to him at 11 A.M. on 4th July 1886"))

p = re.compile('\d+')
print(p.findall("I went to him at 11 A.M. on 4th July 1886"))

['1', '1', '4', '1', '8', '8', '6']
['11', '4', '1886']

Example 3: Word and non-word characters

• \w matches a single word character.
• \w+ matches a group of word characters.
• \W matches non-word characters.

import re

p = re.compile('\w')
print(p.findall("He said * in some_lang."))

p = re.compile('\w+')
print(p.findall("I went to him at 11 A.M., he \
said *** in some_language."))

p = re.compile('\W')
print(p.findall("he said *** in some_language."))

['H', 'e', 's', 'a', 'i', 'd', 'i', 'n', 's', 'o', 'm', 'e', '_', 'l', 'a', 'n', 'g']
['I', 'went', 'to', 'him', 'at', '11', 'A', 'M', 'he', 'said', 'in', 'some_language']
[' ', ' ', '*', '*', '*', ' ', ' ', '.']

Example 4: The regular expression pattern 'ab*' to find and list all occurrences of 'ab' followed by zero or more 'b' characters. In the input string "ababbaabbb". It returns the following list of matches: ['ab', 'abb', 'abbb'].

import re
p = re.compile('ab*')
print(p.findall("ababbaabbb"))

['ab', 'abb', 'a', 'abbb']

Explanation:

• Output 'ab', is valid because of single 'a' accompanied by single 'b'.
• Output 'abb', is valid because of single 'a' accompanied by 2 'b'.
• Output 'a', is valid because of single 'a' accompanied by 0 'b'.
• Output 'abbb', is valid because of single 'a' accompanied by 3 'b'.

3. re.split() 

Splits a string wherever the pattern matches. The remaining characters are returned as list elements.

Syntax: 

re.split(pattern, string, maxsplit=0, flags=0)

• pattern: Regular expression to match split points.
• string: The input string to split.
• maxsplit (optional): Limits the number of splits. Default is 0 (no limit).
• flags (optional): Apply regex flags like re.IGNORECASE.

Example 1: Splitting by non-word characters or digits

This example demonstrates how to split a string using different patterns like non-word characters (\W+), apostrophes, and digits (\d+).

from re import split

print(split('\W+', 'Words, words , Words'))
print(split('\W+', "Word's words Words"))
print(split('\W+', 'On 12th Jan 2016, at 11:02 AM'))
print(split('\d+', 'On 12th Jan 2016, at 11:02 AM'))

['Words', 'words', 'Words']
['Word', 's', 'words', 'Words']
['On', '12th', 'Jan', '2016', 'at', '11', '02', 'AM']
['On ', 'th Jan ', ', at ', ':', ' AM']

Example 2: Using maxsplit and flags

This example shows how to limit the number of splits using maxsplit, and how flags can control case sensitivity.

import re
print(re.split('\d+', 'On 12th Jan 2016, at 11:02 AM', 1))
print(re.split('[a-f]+', 'Aey, Boy oh boy, come here', flags=re.IGNORECASE))
print(re.split('[a-f]+', 'Aey, Boy oh boy, come here'))

['On ', 'th Jan 2016, at 11:02 AM']
['', 'y, ', 'oy oh ', 'oy, ', 'om', ' h', 'r', '']
['A', 'y, Boy oh ', 'oy, ', 'om', ' h', 'r', '']

Note: In the second and third cases of the above , [a-f]+ splits the string using any combination of lowercase letters from 'a' to 'f'. The re.IGNORECASE flag includes uppercase letters in the match.

4. re.sub()

The re.sub() function replaces all occurrences of a pattern in a string with a replacement string.

Syntax:

re.sub(pattern, repl, string, count=0, flags=0)

• pattern: The regex pattern to search for.
• repl: The string to replace matches with.
• string: The input string to process.
• count (optional): Maximum number of substitutions (default is 0, which means replace all).
• flags (optional): Regex flags like re.IGNORECASE.

Example 1: The following examples show different ways to replace the pattern 'ub' with '~*', using various flags and count values.

import re

# Case-insensitive replacement of all 'ub'
print(re.sub('ub', '~*', 'Subject has Uber booked already', flags=re.IGNORECASE))

# Case-sensitive replacement of all 'ub'
print(re.sub('ub', '~*', 'Subject has Uber booked already'))

# Replace only the first 'ub', case-insensitive
print(re.sub('ub', '~*', 'Subject has Uber booked already', count=1, flags=re.IGNORECASE))

# Replace "AND" with "&", ignoring case
print(re.sub(r'\sAND\s', ' & ', 'Baked Beans And Spam', flags=re.IGNORECASE))

S~*ject has ~*er booked already
S~*ject has Uber booked already
S~*ject has Uber booked already
Baked Beans & Spam

5. re.subn()

re.subn() function works just like re.sub(), but instead of returning only the modified string, it returns a tuple: (new_string, number_of_substitutions)

Syntax:

re.subn(pattern, repl, string, count=0, flags=0)

Example: Substitution with count

This example shows how re.subn() gives both the replaced string and the number of times replacements were made.

import re

# Case-sensitive replacement
print(re.subn('ub', '~*', 'Subject has Uber booked already'))

# Case-insensitive replacement
t = re.subn('ub', '~*', 'Subject has Uber booked already', flags=re.IGNORECASE)
print(t)
print(len(t))      # tuple length
print(t[0])        # modified string

('S~*ject has Uber booked already', 1)
('S~*ject has ~*er booked already', 2)
2
S~*ject has ~*er booked already

6. re.escape()

re.escape() function adds a backslash (\) before all special characters in a string. This is useful when you want to match a string literally, including any characters that have special meaning in regex (like ., *, [, ], etc.).

Syntax:

re.escape(string)

Example: Escaping special characters

This example shows how re.escape() treats spaces, brackets, dashes, and tabs as literal characters.

import re

print(re.escape("This is Awesome even 1 AM"))
print(re.escape("I Asked what is this [a-9], he said \t ^WoW"))

This\ is\ Awesome\ even\ 1\ AM
I\ Asked\ what\ is\ this\ \[a\-9\]\,\ he\ said\ \	\ \^WoW

7. re.search()

The re.search() function searches for the first occurrence of a pattern in a string. It returns a match object if found, otherwise None.

Note: Use it when you want to check if a pattern exists or extract the first match.

Example: Search and extract values

This example searches for a date pattern with a month name (letters) followed by a day (digits) in a sentence.

import re

regex = r"([a-zA-Z]+) (\d+)"
match = re.search(regex, "I was born on June 24")

if match:
    print("Match at index %s, %s" % (match.start(), match.end()))
    print("Full match:", match.group(0))
    print("Month:", match.group(1))
    print("Day:", match.group(2))
else:
    print("The regex pattern does not match.")

Match at index 14, 21
Full match: June 24
Month: June
Day: 24

Meta-characters

Metacharacters are special characters in regular expressions used to define search patterns. The re module in Python supports several metacharacters that help you perform powerful pattern matching.

Below is a quick reference table:

Let's discuss each of these metacharacters in detail:

1. \ - Backslash

The backslash (\) makes sure that the character is not treated in a special way. This can be considered a way of escaping metacharacters.

For example, if you want to search for the dot(.) in the string then you will find that dot(.) will be treated as a special character as is one of the metacharacters (as shown in the above table). So for this case, we will use the backslash(\) just before the dot(.) so that it will lose its specialty. See the below example for a better understanding.

Example:

The first search (re.search(r'.', s)) matches any character, not just the period, while the second search (re.search(r'\.', s)) specifically looks for and matches the period character.

import re

s = 'geeks.forgeeks'

# without using \
match = re.search(r'.', s)
print(match)

# using \
match = re.search(r'\.', s)
print(match)

<re.Match object; span=(0, 1), match='g'>
<re.Match object; span=(5, 6), match='.'>

2. [] - Square Brackets

Square Brackets ([]) represent a character class consisting of a set of characters that we wish to match. For example, the character class [abc] will match any single a, b, or c. 

We can also specify a range of characters using - inside the square brackets. For example, 

• [0, 3] is sample as [0123]
• [a-c] is same as [abc]

We can also invert the character class using the caret(^) symbol. For example, 

• [^0-3] means any character except 0, 1, 2, or 3
• [^a-c] means any character except a, b, or c

Example:

In this code, you're using regular expressions to find all the characters in the string that fall within the range of 'a' to 'm'. The re.findall() function returns a list of all such characters. In the given string, the characters that match this pattern are: 'c', 'k', 'b', 'f', 'j', 'e', 'h', 'l', 'd', 'g'.

import re

string = "The quick brown fox jumps over the lazy dog"
pattern = "[a-m]"
result = re.findall(pattern, string)

print(result) 

['h', 'e', 'i', 'c', 'k', 'b', 'f', 'j', 'm', 'e', 'h', 'e', 'l', 'a', 'd', 'g']

3. ^ - Caret

Caret (^) symbol matches the beginning of the string i.e. checks whether the string starts with the given character(s) or not. For example -  

• ^g will check if the string starts with g such as geeks, globe, girl, g, etc.
• ^ge will check if the string starts with ge such as geeks, geeksforgeeks, etc.

Example:

This code uses regular expressions to check if a list of strings starts with "The". If a string begins with "The," it's marked as "Matched" otherwise, it's labeled as "Not matched".

import re
regex = r'^The'
strings = ['The quick brown fox', 'The lazy dog', 'A quick brown fox']
for string in strings:
    if re.match(regex, string):
        print(f'Matched: {string}')
    else:
        print(f'Not matched: {string}')

Matched: The quick brown fox
Matched: The lazy dog
Not matched: A quick brown fox

4. $ - Dollar

Dollar($) symbol matches the end of the string i.e checks whether the string ends with the given character(s) or not. For example-

• s$ will check for the string that ends with a such as geeks, ends, s, etc.
• ks$ will check for the string that ends with ks such as geeks, geeksforgeeks, ks, etc.

Example:

This code uses a regular expression to check if the string ends with "World!". If a match is found, it prints "Match found!" otherwise, it prints "Match not found".

import re

string = "Hello World!"
pattern = r"World!$"

match = re.search(pattern, string)
if match:
    print("Match found!")
else:
    print("Match not found.")

Match found!

5. . - Dot

Dot(.) symbol matches only a single character except for the newline character (\n). For example -  

• a.b will check for the string that contains any character at the place of the dot such as acb, acbd, abbb, etc
• .. will check if the string contains at least 2 characters

Example:

This code uses a regular expression to search for the pattern "brown.fox" within the string. The dot (.) in the pattern represents any character. If a match is found, it prints "Match found!" otherwise, it prints "Match not found".

import re

string = "The quick brown fox jumps over the lazy dog."
pattern = r"brown.fox"

match = re.search(pattern, string)
if match:
    print("Match found!")
else:
    print("Match not found.")

Match found!

6. | - Or

The | operator means either pattern on its left or right can match.

• a|b will match any string that contains a or b such as acd, bcd, abcd, etc.

7. ? - Question Mark

The question mark (?) indicates that the preceding element should be matched zero or one time. It allows you to specify that the element is optional, meaning it may occur once or not at all. For example,

• ab?c will be matched for the string ac, acb, dabc but will not be matched for abbc because there are two b. Similarly, it will not be matched for abdc because b is not followed by c.

8.* - Star

Star (*) symbol matches zero or more occurrences of the regex preceding the * symbol. For example -  

• ab*c will be matched for the string ac, abc, abbbc, dabc, etc. but will not be matched for abdc because b is not followed by c.

9. + - Plus

Plus (+) symbol matches one or more occurrences of the regex preceding the + symbol. For example -  

• ab+c will be matched for the string abc, abbc, dabc, but will not be matched for ac, abdc, because there is no b in ac and b, is not followed by c in abdc.

10. {m, n} - Braces

Braces match any repetitions preceding regex from m to n both inclusive. For example -  

• a{2, 4} will be matched for the string aaab, baaaac, gaad, but will not be matched for strings like abc, bc because there is only one a or no a in both the cases.

11. (<regex>) - Group

Group symbol is used to group sub-patterns. For example -  

• (a|b)cd will match for strings like acd, abcd, gacd, etc.

Special Sequences

Special sequences do not match for the actual character in the string instead it tells the specific location in the search string where the match must occur. It makes it easier to write commonly used patterns.  

List of special sequences 

Sets for character matching

A Set is a set of characters enclosed in '[]' brackets. Sets are used to match a single character in the set of characters specified between brackets. Below is the list of Sets:

Match Object

A Match object contains all the information about the search and the result and if there is no match found then None will be returned. Let's see some of the commonly used methods and attributes of the match object.

1. Getting the string and the regex

match.re attribute returns the regular expression passed and match.string attribute returns the string passed.

Example: Getting the string and the regex of the matched object

The code searches for the letter "G" at a word boundary in the string "Welcome to GeeksForGeeks" and prints the regular expression pattern (res.re) and the original string (res.string).

import re
s = "Welcome to GeeksForGeeks"
res = re.search(r"\bG", s)

print(res.re)
print(res.string)

re.compile('\\bG')
Welcome to GeeksForGeeks

2. Getting index of matched object

• start() method returns the starting index of the matched substring
• end() method returns the ending index of the matched substring
• span() method returns a tuple containing the starting and the ending index of the matched substring

Example: Getting index of matched object 

The code searches for the substring "Gee" at a word boundary in the string "Welcome to GeeksForGeeks" and prints the start index of the match (res.start()), the end index of the match (res.end()), and the span of the match (res.span()).

import re

s = "Welcome to GeeksForGeeks"

res = re.search(r"\bGee", s)

print(res.start())
print(res.end())
print(res.span())

11
14
(11, 14)

3. Getting matched substring

group() method returns the part of the string for which the patterns match. See the below example for a better understanding.

Example: Getting matched substring 

The code searches for a sequence of two non-digit characters followed by a space and the letter 't' in the string "Welcome to GeeksForGeeks" and prints the matched text using res.group().

import re
s = "Welcome to GeeksForGeeks"
res = re.search(r"\D{2} t", s)
print(res.group())

me t

In the above example, our pattern specifies for the string that contains at least 2 characters which are followed by a space, and that space is followed by a t. 

Basic RegEx Patterns

Let's understand some of the basic regular expressions. They are as follows:

1. Character Classes

Character classes allow matching any one character from a specified set. They are enclosed in square brackets [].

import re
print(re.findall(r'[Gg]eeks', 'GeeksforGeeks: \
                 A computer science portal for geeks'))

['Geeks', 'Geeks', 'geeks']

2. Ranges

In RegEx, a range allows matching characters or digits within a span using - inside []. For example, [0-9] matches digits, [A-Z] matches uppercase letters.

import re
print('Range',re.search(r'[a-zA-Z]', 'x'))

Range <re.Match object; span=(0, 1), match='x'>

3. Negation

Negation in a character class is specified by placing a ^ at the beginning of the brackets, meaning match anything except those characters.

Syntax:

[^a-z]

Example:

import re

print(re.search(r'[^a-z]', 'c'))
print(re.search(r'G[^e]', 'Geeks')) 

None
None

3. Shortcuts

Shortcuts are shorthand representations for common character classes. Let's discuss some of the shortcuts provided by the regular expression engine.

• \w - matches a word character
• \d - matches digit character
• \s - matches whitespace character (space, tab, newline, etc.)
• \b - matches a zero-length character

import re

print('Geeks:', re.search(r'\bGeeks\b', 'Geeks'))
print('GeeksforGeeks:', re.search(r'\bGeeks\b', 'GeeksforGeeks'))

Geeks: <_sre.SRE_Match object; span=(0, 5), match='Geeks'>
GeeksforGeeks: None

4. Beginning and End of String

The ^ character chooses the beginning of a string and the $ character chooses the end of a string.

import re


# Beginning of String
match = re.search(r'^Geek', 'Campus Geek of the month')
print('Beg. of String:', match)

match = re.search(r'^Geek', 'Geek of the month')
print('Beg. of String:', match)

# End of String
match = re.search(r'Geeks$', 'Compute science portal-GeeksforGeeks')
print('End of String:', match)

Beg. of String: None
Beg. of String: <_sre.SRE_Match object; span=(0, 4), match='Geek'>
End of String: <_sre.SRE_Match object; span=(31, 36), match='Geeks'>

5. Any Character

The . character represents any single character outside a bracketed character class.

import re

print('Any Character', re.search(r'p.th.n', 'python 3'))

Any Character <_sre.SRE_Match object; span=(0, 6), match='python'>

6. Optional Characters

Regular expression engine allows you to specify optional characters using the ? character. It allows a character or character class either to present once or else not to occur. Let's consider the example of a word with an alternative spelling - color or colour.

import re

print('Color',re.search(r'colou?r', 'color')) 
print('Colour',re.search(r'colou?r', 'colour'))

Color <_sre.SRE_Match object; span=(0, 5), match='color'>
Colour <_sre.SRE_Match object; span=(0, 6), match='colour'>

7. Repetition

Repetition enables you to repeat the same character or character class. Consider an example of a date that consists of day, month, and year. Let's use a regular expression to identify the date (mm-dd-yyyy).

import re


print('Date{mm-dd-yyyy}:', re.search(r'[\d]{2}-[\d]{2}-[\d]{4}',
                                     '18-08-2020'))

Date{mm-dd-yyyy}: <_sre.SRE_Match object; span=(0, 10), match='18-08-2020'>

Here, the regular expression engine checks for two consecutive digits. Upon finding the match, it moves to the hyphen character. After then, it checks the next two consecutive digits and the process is repeated.  

Let's discuss three other regular expressions under repetition.

7.1 Repetition ranges

The repetition range is useful when you have to accept one or more formats. Consider a scenario where both three digits, as well as four digits, are accepted. Let's have a look at the regular expression.

import re


print('Three Digit:', re.search(r'[\d]{3,4}', '189'))
print('Four Digit:', re.search(r'[\d]{3,4}', '2145'))

Three Digit: <_sre.SRE_Match object; span=(0, 3), match='189'>
Four Digit: <_sre.SRE_Match object; span=(0, 4), match='2145'>

7.2 Open-Ended Ranges

There are scenarios where there is no limit for a character repetition. In such scenarios, you can set the upper limit as infinitive. A common example is matching street addresses. Let's have a look  

import re


print(re.search(r'[\d]{1,}','5th Floor, A-118,\
Sector-136, Noida, Uttar Pradesh - 201305'))

<_sre.SRE_Match object; span=(0, 1), match='5'>

7.3 Shorthand

Shorthand characters allow you to use + character to specify one or more ({1,}) and * character to specify zero or more ({0,}.

import re

print(re.search(r'[\d]+', '5th Floor, A-118,\
Sector-136, Noida, Uttar Pradesh - 201305'))

<_sre.SRE_Match object; span=(0, 1), match='5'>

8. Grouping

Grouping is the process of separating an expression into groups by using parentheses, and it allows you to fetch each individual matching group.  

import re


grp = re.search(r'([\d]{2})-([\d]{2})-([\d]{4})', '26-08-2020')
print(grp)

<_sre.SRE_Match object; span=(0, 10), match='26-08-2020'>

Let's see some of its functionality.

8.1 Return the entire match

The re module allows you to return the entire match using the group() method

import re


grp = re.search(r'([\d]{2})-([\d]{2})-([\d]{4})','26-08-2020')
print(grp.group())

26-08-2020

8.2 Return a tuple of matched groups

You can use groups() method to return a tuple that holds individual matched groups

import re


grp = re.search(r'([\d]{2})-([\d]{2})-([\d]{4})','26-08-2020')
print(grp.groups())

('26', '08', '2020')

8.3 Retrieve a single group

Upon passing the index to a group method, you can retrieve just a single group.

import re


grp = re.search(r'([\d]{2})-([\d]{2})-([\d]{4})','26-08-2020')
print(grp.group(3))

2020

8.4 Name your groups

The re module allows you to name your groups. Let's look into the syntax.

import re


match = re.search(r'(?P<dd>[\d]{2})-(?P<mm>[\d]{2})-(?P<yyyy>[\d]{4})',
                  '26-08-2020')
print(match.group('mm'))

08

8.5 Individual match as a dictionary

We have seen how regular expression provides a tuple of individual groups. Not only tuple, but it can also provide individual match as a dictionary in which the name of each group acts as the dictionary key.

import re


match = re.search(r'(?P<dd>[\d]{2})-(?P<mm>[\d]{2})-(?P<yyyy>[\d]{4})',
                  '26-08-2020')
print(match.groupdict())

{'dd': '26', 'mm': '08', 'yyyy': '2020'}

9. Lookahead

In the case of a  negated character class, it won't match if a character is not present to check against the negated character. We can overcome this case by using lookahead; it accepts or rejects a match based on the presence or absence of content.  

import re


print('negation:', re.search(r'n[^e]', 'Python'))
print('lookahead:', re.search(r'n(?!e)', 'Python'))

negation: None
lookahead: <_sre.SRE_Match object; span=(5, 6), match='n'>

Lookahead can also disqualify the match if it is not followed by a particular character. This process is called a positive lookahead, and can be achieved by simply replacing ! character with = character.

import re

print('positive lookahead', re.search(r'n(?=e)', 'jasmine'))

positive lookahead <_sre.SRE_Match object; span=(5, 6), match='n'>

10. Substitution

The regular expression can replace the string and returns the replaced one using the re.sub method. It is useful when you want to avoid characters such as /, -, ., etc. before storing it to a database. It takes three arguments:

• the regular expression
• the replacement string
• the source string being searched

Let's have a look at the below code that replaces - character from a credit card number.

import re

print(re.sub(r'([\d]{4})-([\d]{4})-([\d]{4})-([\d]{4})',r'\1\2\3\4',
             '1111-2222-3333-4444'))

1111222233334444

Related Article:

Regular Expressions in Python - Set 2