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'Copy

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)Copy

Output:

['simple', 'complex']Copy

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 \dcharacter set with the quantifiers{}:

'\d{2}:\d{2}'Copy

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]Copy

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]\dCopy

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'Copy

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())Copy

Output:

09
22:30Copy

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)Copy

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]\dCopy

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())Copy

Output:

09:30
22:30Copy

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.

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