Regular Expression Cheatsheet

\d matches any single digit and is the same as the character class [0-9]. Its opposite, \D, matches any non-digit character. So \d\d matches a pair of digits like 42.

Both repeat the token before them, but * matches zero or more, so it can match nothing at all, while + matches one or more and needs at least one occurrence. That is why ab*c matches ac, but ab+c does not.

By default * and + are greedy and grab as much as they can. Adding a ? makes them lazy so they match as little as possible, which is why <.*?> stops at the first > and matches only <a> in <a><b>.

A lookahead like (?=...) asserts a pattern follows the current spot without consuming it, and (?!...) asserts one does not. Lookbehinds (?<=...) and (?<!...) do the same for what comes before, so (?<=\$)\d+ matches 50 in $50.

The g flag returns every match rather than just the first, i makes matching case-insensitive, m lets ^ and $ match at line breaks, and s (dotall) lets the dot match newlines too. The sheet also lists u for Unicode and y for sticky matching.

Escape it with a backslash. So \. matches a real dot rather than any character, and \\ matches a single backslash, which is handy for Windows paths like C:\\path.

It does a case-insensitive substring match against both the token and its meaning, so searching digit also surfaces \w because its meaning mentions digits. Clearing the box brings back the full sheet grouped by category.

Most are, but a couple are flagged as reference only. \A and \z are start and end of input anchors from other engines like PCRE and are not supported in JavaScript, where you use ^ and $ instead.

(?<name>...) captures into a name you choose, and \k<name> refers back to it later in the pattern. Numbered backreferences also exist, so (\w)\1 matches a doubled letter such as the oo in book.