Groups, alternation and capturing

Combine options, capture parts of a match and reuse them.

Parentheses group parts of a pattern and capture what they match. The pipe | means "or".

cat|dog                      # cat or dog
(Mr|Ms)\.? \w+              # Mr Smith, Ms. Lee
(\d{4})-(\d{2})-(\d{2})     # captures year, month and day of 2026-10-04

Captured groups can be reused in replacements. In JavaScript, "2026-10-04".replace(/(\d{4})-(\d{2})-(\d{2})/, "$3/$2/$1") gives 04/10/2026.

Non-capturing and named groups

• (?:...) groups without capturing.
• (?<year>\d{4}) names a group (supported in JavaScript, Python as (?P<year>...), and others).

Flags

• i ignore case.
• g find all matches (JavaScript).
• m make ^ and $ match at each line.

A word on email validation

NoteA simple pattern like ^[^@\s]+@[^@\s]+\.[^@\s]+$ catches obvious typos. The only real way to validate an email is to send a message to it.

CarefulBadly written patterns with nested quantifiers, such as (a+)+$, can take exponential time on some inputs (ReDoS). Keep patterns simple and avoid running untrusted patterns.