Python Regex Character Set

Summary: in this tutorial, you learn about character sets in regular expressions including digits, words, whitespace, and the dot (.).

Introduction to Python regex character sets

A character set (or a character class) is a set of characters, for example, digits (from 0 to 9), alphabets (from a to z), and whitespace.

A character set allows you to construct regular expressions with patterns that match a string with one or more characters in a set.

\d: digit character set

Regular expressions use \d to represent a digit character set that matches a single digit from 0 to 9.

The following example uses the finditer() function to match every single digit in a string using the \d character set:

import re

s = 'Python 3.0 was released in 2008'
matches = re.finditer('\d', s)
for match in matches:
    print(match.group())Code language: Python (python)

Output:

3
0
2
0
0
8Code language: Python (python)

To match a group of two digits, you use the \d\d. For example:

import re

s = 'Python 3.0 was released in 2008'
matches = re.finditer('\d\d', s)
for match in matches:
    print(match.group())Code language: Python (python)

Output:

20
08Code language: Python (python)

Similarly, you can match a group of four digits using the \d\d\d\d pattern:

import re

s = 'Python 3.0 was released in 2008'
matches = re.finditer('\d\d\d\d', s)
for match in matches:
    print(match.group())Code language: Python (python)

Output:

2008Code language: Python (python)

Later, you’ll learn how to use quantifiers to shorten the pattern. So instead of using the \d\d\d\d pattern, you can use the shorter one like \d{4}

\w: the word character set

Regular expressions use \w to represent the word character set. The \w matches a single ASCII character including Latin alphabet, digit, and underscore (_).

The following example uses the finditer() function to match every single word character in a string using the \w character set:

import re

s = 'Python 3.0'
matches = re.finditer('\w', s)
for match in matches:
    print(match.group())Code language: Python (python)

Output:

P
y
t
h
o
n
3
0Code language: Python (python)

Notice that the whitespace and . are not included in the matches.

\s : whitespace character set

The \s matches whitespace including a space, a tab, a newline, a carriage return, and a vertical tab.

The following example uses the whitespace character set to match a space in a string:

import re

s = 'Python 3.0'
matches = re.finditer('\s', s)
for match in matches:
    print(match)Code language: Python (python)

Output:

<re.Match object; span=(6, 7), match=' '>Code language: Python (python)

Inverse character sets

A character set has an inverse character set that uses the same letter but in uppercase. The following table shows the character sets and their inverse ones:

The following example uses the \D to match the non-digit from a phone number:

import re

phone_no = '+1-(650)-513-0514'
matches = re.finditer('\D', phone_no)
for match in matches:
    print(match.group())Code language: Python (python)

Output:

+
-
(
)
-
-Code language: Python (python)

To turn the phone number +1-(650)-513-0514 into the 16505130514, you can use the sub() function:

import re

phone_no = re.sub('\D', '', '+1-(650)-513-0514')
print(phone_no)Code language: Python (python)

Output:

16505130514Code language: Python (python)

In this example, the sub() function replaces the character that matches the pattern \D with the literal string '' in the formatted phone number.

The dot(.) character set

The dot (.) character set matches any single character except the new line (\n). The following example uses the dot (.) character set to match every single character but the new line:

import re

version = "Python\n4"
matches = re.finditer('.', version)
for match in matches:
    print(match.group())Code language: Python (python)

Output:

P
y
t
h
o
n
4Code language: Python (python)

Summary

• Use \d character set to match any single digit.
• Use \w character set to match any single word character.
• Use \s character set to match any whitespace.
• The \D, \W, \S character set are the inverse sets of \d, \w, and \s character set.
• Use the dot character set (.) to match any character but a new line.