Python Regex Alternation

Summary: in this tutorial, you’ll learn about Python regex alternation, which behaves like the “OR” operator in regular expressions.

Introduction to the Python regex alternation

To represent an alternation in regular expressions, you use the pipe operator (|). The pipe operator is called the alternation. It is like the or operator in Python.

The following regular expression uses an alternation to match either the literal string complex and simple:

'simple|complex'Code language: Python (python)

For example, the following program uses the above regular expression to match either the literal string simple or complex:

import re

s = 'simple is better than complex'
pattern = r'simple|complex'

matches = re.findall(pattern,s)
print(matches)Code language: Python (python)

Output:

['simple', 'complex']Code language: Python (python)

Python regex alternation examples

Let’s take more examples of using the regex alternation.

1) Use Python regex alternation for matching time in hh:mm format

To match a time string in the hh:mm format, you can combine the \d character set with the quantifiers {}:

'\d{2}:\d{2}'Code language: Python (python)

In this pattern:

• \d{2} matches two digits.
• : matches the colon character.
• \d{2} matches two digits.

However, the rule \d{2} also matches a number that is not a valid hour or minute, such as 99.

To fix this, you can use the regex alternation.

If the valid hour ranges from 01 to 23, you can use the following pattern to match the hour part:

[01]\d|2[0-3]Code language: Python (python)

In this pattern:

• [01] matches a single digit 0 or 1
• \d matches a single digit from 0 to 9
• [01]\d matches 00, 01 to 19
• 2 matches the digit 2
• [0-3] matches a single digit from 0 to 3 including 0, 1, 2, 3
• 2[0-3] matches two digits 20, 21, 22, and 23.

Therefore, the [01]\d|2[0-3] matches two digits from 00 to 23

Because the valid minute ranges from 00 to 59, you can use the following pattern to match it:

[0-5]\dCode language: Python (python)

The following regular expression combines the two rules above to match the time in the hh:mm format:

'[01]\d|2[0-3]:[0-5]\d'Code language: Python (python)

However, this regular expression will not work as expected. For example:

import re

s = '09:30 30:61 22:30 25:99'
pattern = r'[01]\d|2[0-3]:[0-5]\d'

matches = re.finditer(pattern, s)
for match in matches:
    print(match.group())Code language: Python (python)

Output:

09
22:30Code language: Python (python)

In this example, the regex engine treats pattern [01]\d|2[0-3]:[0-5]\d as two main parts separated by the alternation:

[01]\d
OR
2[0-3]):([0-5]\d)Code language: Python (python)

To fix it, you need to wrap the alternation inside parentheses to indicate that only that part is alternated, not the whole expression like this:

([01]\d|2[0-3]):[0-5]\dCode language: Python (python)

Now, the program works as expected:

import re

s = '09:30 30:61 22:30 25:99'
pattern = r'([01]\d|2[0-3]):[0-5]\d'

matches = re.finditer(pattern, s)
for match in matches:
    print(match.group())Code language: Python (python)

Output:

09:30
22:30Code language: Python (python)

Summary

• The regex alternation X | Y matches either X or Y.
• The regex alternation is like an OR operator in regular expressions.
• Place the alternation part inside parentheses () to express that only that part is alternated.