regex --library
31 patterns for things you would otherwise look up again. Each one is checked against the examples next to it on every build, and each says where it stops being reliable.
/^[^\s@]+@[^\s@]+\.[^\s@]{2,}$/Something, an @, a domain with a dot. Deliberately loose.
No pattern can tell whether a mailbox exists. Use this to catch typos, then send a confirmation mail.
/^https?:\/\/[^\s/$.?#][^\s]*$/iA web address with scheme. Anything without spaces may follow the host.
/^(?=.{1,253}$)(?:[a-z0-9](?:[a-z0-9-]{0,61}[a-z0-9])?\.)+[a-z]{2,63}$/iLabels of letters, digits and hyphens, no hyphen at the start or end of a label, at most 253 characters.
/^(?:(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)\.){3}(?:25[0-5]|2[0-4]\d|1\d\d|[1-9]?\d)$/Four numbers from 0 to 255, no leading zeros.
/^(?:(?:[0-9a-f]{1,4}:){7}[0-9a-f]{1,4}|(?:[0-9a-f]{1,4}:){1,7}:|(?:[0-9a-f]{1,4}:){1,6}:[0-9a-f]{1,4}|(?:[0-9a-f]{1,4}:){1,5}(?::[0-9a-f]{1,4}){1,2}|(?:[0-9a-f]{1,4}:){1,4}(?::[0-9a-f]{1,4}){1,3}|(?:[0-9a-f]{1,4}:){1,3}(?::[0-9a-f]{1,4}){1,4}|(?:[0-9a-f]{1,4}:){1,2}(?::[0-9a-f]{1,4}){1,5}|[0-9a-f]{1,4}:(?::[0-9a-f]{1,4}){1,6}|:(?:(?::[0-9a-f]{1,4}){1,7}|:))$/iEight groups of hex digits, with :: standing in for one run of zero groups.
Does not cover the mixed form with an IPv4 tail (::ffff:192.0.2.1) or zone ids (%eth0).
/^[0-9a-f]{2}([:-])(?:[0-9a-f]{2}\1){4}[0-9a-f]{2}$/iSix hex pairs. The backreference makes sure the same separator is used throughout.
/^[a-z0-9]+(?:-[a-z0-9]+)*$/Lowercase letters and digits, single hyphens between words.
/^#(?:[0-9a-f]{3,4}|[0-9a-f]{6}|[0-9a-f]{8})$/iCSS hex colors with 3, 4, 6 or 8 digits.
/^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{12,}$/At least 12 characters with a lowercase letter, an uppercase letter and a digit. One lookahead per rule.
Length protects better than character rules. If you can choose the policy, ask for length.
/^-?(?:0|[1-9]\d*)$/Whole number with optional minus, no leading zeros.
/^-?(?:0|[1-9]\d*)(?:\.\d+)?$/Integer with an optional fraction after a dot.
/^-?(?:0|[1-9]\d*)(?:\.\d+)?(?:[eE][+-]?\d+)?$/The number grammar of JSON: optional fraction, optional exponent.
/^-?\d{1,3}(?:,\d{3})+(?:\.\d+)?$/Groups of three digits separated by commas, optional fraction.
/^(?:0x)?[0-9a-f]+$/iHex digits with an optional 0x prefix.
/^(?<major>0|[1-9]\d*)\.(?<minor>0|[1-9]\d*)\.(?<patch>0|[1-9]\d*)(?:-(?<pre>[0-9A-Za-z-]+(?:\.[0-9A-Za-z-]+)*))?(?:\+(?<build>[0-9A-Za-z-]+(?:\.[0-9A-Za-z-]+)*))?$/major.minor.patch with optional pre-release and build metadata, each part as a named group.
Slightly looser than the spec: numeric pre-release parts may have leading zeros.
/^\d{4}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[12]\d|3[01])$/YYYY-MM-DD with month 01 to 12 and day 01 to 31.
Accepts 2026-02-31. A pattern cannot know the calendar; parse the date in code to be sure.
/^(?:[01]\d|2[0-3]):[0-5]\d(?::[0-5]\d)?$/HH:MM with optional seconds.
/^\d{4}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[12]\d|3[01])T(?:[01]\d|2[0-3]):[0-5]\d:[0-5]\d(?:\.\d+)?(?:Z|[+-](?:[01]\d|2[0-3]):[0-5]\d)$/Date, T, time, optional fraction of a second, then Z or a UTC offset. The shape of Date.toISOString().
/^(?:0?[1-9]|[12]\d|3[01])\.(?:0?[1-9]|1[0-2])\.\d{4}$/The format used in Germany and much of Europe, with or without leading zeros.
/\b(\w+)\s+\1\b/giThe same word twice in a row. Replace with $1 to remove the copy.
This is is a test of the the regex tester. Nothing wrong here.
/[ \t]+$/gmSpaces and tabs at the end of a line. Replace with nothing to clean up.
clean line trailing spaces tab at the end last line
/^[ \t]*\r?\n/gmLines that are empty or hold only spaces. Replace with nothing to remove them.
first second
/"(?:[^"\\]|\\.)*"/gDouble-quoted text in which \" does not end the string.
say "hello", then "a \"quoted\" word" and "".
/(?<=[a-z0-9])(?=[A-Z])/gThe empty positions where a new word starts. Replace with a space, - or _ to split an identifier.
parseHttpResponse2Json
/\[([^\]]+)\]\(([^)\s]+)\)/gInline links, with the label in group 1 and the target in group 2.
See [reez.cc](https://reez.cc) and [the docs](/docs/intro), not [this].
/^[0-9a-f]{8}-[0-9a-f]{4}-[1-8][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/iVersions 1 to 8 in the usual 8-4-4-4-12 form.
/^[A-Za-z0-9_-]{2,}\.[A-Za-z0-9_-]{2,}\.[A-Za-z0-9_-]*$/Three base64url parts separated by dots. Checks the shape only, not the signature.
/<\/?([a-z][a-z0-9-]*)\b[^>]*>/giOpening, closing and self-closing tags, with the tag name in group 1.
<p class="lead">Hi <br/> there</p>
Fine for a quick search. Do not parse HTML with it: a > inside an attribute value ends the match early.
/(?<!:)\/\/.*$/gmFrom // to the end of the line. The lookbehind skips the // in URLs.
const port = 8080; // default const url = "https://reez.cc"; // home
Does not know about strings: a // inside quotes that is not part of a URL still matches.
/\/\*[\s\S]*?\*\//gFrom /* to the next */, across lines. The lazy quantifier stops at the first */.
/* one */ code(); /* two lines */ more();
/^([A-Z_][A-Z0-9_]*)=(.*)$/gmLines of a .env file, with the name in group 1 and the value in group 2.
PORT=8080 # comment DEBUG= lower=ignored