Introduction to the regex alternation

Regex uses the pipe operator (|) to represent an alternation, which is like the logical OR operator in regular expressions. The alternation allows you to match either A or B:

The following example uses the alternation to match either the JavaScript or JS in the string 'JavaScript and JS':

Code language: JavaScript (javascript)

Output:

Code language: plaintext (plaintext)

Regex alternation examples

The following example illustrates the practical applications of the regex alternation.

1) Using regex alternation to match time string in the hh:mm format

The following regular expression that combines the \d character set with the quantifiers {} to match a time string in the format hh:mm:

Code language: JavaScript (javascript)

In this regular expression:

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

But the rule \d{2} also matches an invalid hour or minute for example 99. To make it match more precisely, you can use an alternation.

Since the valid hours are from 01 to 23, you can use the following pattern to match the hour part:

Code language: JavaScript (javascript)

In this pattern:

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

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

Similarly, you can use the following rule to match a valid minute that ranges from 00 to 59:

Code language: JavaScript (javascript)

The following regular expression combines those rules to match a time string in the hh:mm format:

Code language: JavaScript (javascript)

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

Code language: JavaScript (javascript)

Output:

Code language: JavaScript (javascript)

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

Code language: Python (python)

To fix it, you use parentheses to wrap the alternation. It indicates that only the wrapped part is alternated, not the entire pattern:

Code language: JavaScript (javascript)

Now, the script works as expected:

Code language: JavaScript (javascript)

Output:

Code language: JavaScript (javascript)

Summary

• The alternation A | B matches either A or B.
• The alternation is like an OR operator in regular expressions.
• Use parentheses () to wrap the parts that you want to apply the alternation.