Groups and Alternation

Groups and Alternation

Once you can match single characters, grouping lets you treat several elements as a unit so you can apply quantifiers or alternatives to the whole group.

Capturing Groups

Parentheses () create a capturing group. The text it matches is stored and can be referenced later — by backreference or by index in the match result.

/(\d{4})-(\d{2})-(\d{2})/
on "2024-07-05": group 1 = "2024", 2 = "07", 3 = "05"

Non-Capturing Groups

When you only need grouping to apply a quantifier — not the captured text — use (?:...). It avoids filling capture slots and is slightly faster.

/(?:https?|ftp):\/\//
matches "http://", "https://", or "ftp://"

Alternation

The pipe | means OR. It has the lowest precedence, so cat|dog matches "cat" or "dog", while ca(t|d) matches "cat" or "cad". Parentheses control the scope.

/apple|orange/   "apple" or "orange"
/gr(a|e)y/      "gray" or "grey"

Alternation is not a character class. [aeiou] matches one of several single characters; cat|dog|bird matches one of several whole alternatives. Use classes for single characters, alternation for multi-character choices.

Backreferences

A backreference \1, \2, and so on matches the exact text an earlier capturing group captured. This lets you detect repeats such as doubled words or matching quote characters.

/(\w+)\s\1/      a repeated word: "the the"
/(['"]).*?\1/    quoted string with a matching quote

Named Groups

For readability, name your groups with (?<name>...) and refer back with \k<name>. Named groups make complex patterns far easier to maintain.

/(?<year>\d{4})-(?<month>\d{2})/