Language cheatsheet

Regular expressions pcre2 Cheatsheet

Regex patterns for matching, grouping, lookarounds and common formats, plus how to use them in grep, sed, JavaScript and Python.

Characters and anchors

Match digits — Match one or more digits. Use \D for anything that is not a digit.
Match word characters — Match letters, digits and underscore. Use \W for the opposite.
Match whitespace — Match spaces, tabs and line breaks. Use \S for the opposite.
Match any character — Match any single character except a line break, unless the dotall flag is set.
Match one of several characters — Match a single letter, either case.
Match anything except some characters — Match one character that is not a digit.
Match the whole line — Anchor to the start and end of the text. This matches a phone number like 555-1234.
Match a whole word — Match cat but not category or concat.
Match a special character literally — Escape characters such as . * + ? ( ) [ ] { } | \ ^ $ with a backslash.

Repeat a pattern — Match between two and four letters a. Also: * zero or more, + one or more, ? optional.
Match as little as possible — Add ? after a quantifier to make it lazy. This matches the shortest text between the quotes.
Match one of several options — Match either cat or dog, whichever appears.
Capture parts of a match — Capture the year, month and day of an ISO date as groups 1, 2 and 3.
Group without capturing — Group for alternation or repetition without creating a numbered group.
Name a capture group — Capture a year by name. JavaScript, PCRE and Java use (?<name>...); Python uses (?P<name>...).
Match repeated text — Match a repeated word such as the the, using a backreference to the first group.
Ignore case — Match hello in any case. Many tools use an i flag instead.

Match only if followed by something — Match a number only when it is followed by px, without including px in the match.
Match only if not followed by something — Match a number that is not followed by px.
Match only if preceded by something — Match the digits after a dollar sign, without including the sign.
Match only if not preceded by something — Match a number that does not come right after a dollar sign.
Require several conditions at once — Use lookaheads at the start to require a lowercase letter, an uppercase letter, a digit and 8 or more characters.

Email address (simple) — A permissive check for [email protected]. Fully validating email is impractical, so confirm addresses by sending a message.
Web URL — Match an http or https URL up to the next whitespace.
IPv4 address (shape only) — Match four groups of 1 to 3 digits. It doesn't check that each is 255 or less.
ISO date — Match dates like 2026-10-30. It doesn't check that the month or day are valid.
Hex colour — Match both the short #fff and the long #ffffff forms.
UUID — Match a UUID in the standard 8-4-4-4-12 format.
Semantic version — Match versions like 1.4.2, 2.0.0-beta.1 or 1.0.0+build.5.
URL slug — Match lowercase words joined by single hyphens.
Hashtag — Match a hash sign followed by word characters.
Double-quoted string with escapes — Match a quoted string that may contain escaped quotes.
Trim whitespace — Match leading and trailing whitespace. Replace matches with nothing to trim.
Collapse repeated spaces — Match two or more whitespace characters, to replace with one space.

Search with grep — Print lines made only of digits. -E enables extended regular expressions.
Use lookarounds with grep — Print only the matching part, using Perl-compatible syntax with -P (GNU grep).
Rewrite text with sed — Swap two numbers around a hyphen, using capture groups in the replacement.
Match in JavaScript — Extract the parts of a date with a regular expression literal.
Replace in JavaScript — Replace every run of whitespace with a single space. The g flag replaces all matches.
Find all matches in Python — Return every run of digits in a string. Use raw strings so backslashes aren't treated as escapes.
Replace in Python — Replace matches using a pattern and a replacement, with \1 style references.

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