Regex Tester

The five flags offered and what each one changes

The sticky flag y, the indices flag d and the newer v flag are not offered. The flags are handed to the engine exactly as shown after the closing slash, so turning g off is a real change rather than a cosmetic one: the search stops at the first hit, the count reads 1 or 0, and the label switches to First match only. That mirrors what exec and String.prototype.match do with a non-global expression in your own code.

How matches are found and shown

The pattern and flags are handed to the JavaScript RegExp constructor. With g on, the expression runs in an exec loop over the test string; with g off, exec is called once and the loop ends after that first hit. Each hit is wrapped in a highlight in the preview and added to the table. An empty match (for example from the pattern a*) advances one character so a global loop can finish, and a guard stops after 10,000 matches. The table lists the first 100 matches; the count in the Matches label still reports the full total.

The Index column is the zero-based position of the match in the test string. With the default pattern \w+@\w+\.\w+ and the sample sentence, the first address hello@alldaytoolkit.com starts at index 14 and support@example.org at index 41. The Groups column joins the capture groups with commas: change the pattern to (\w+)@(\w+)\.(\w+) and the first row reads hello, alldaytoolkit, com. A pattern with no groups shows a dash, and a group that did not take part in the match shows as an empty entry. Named groups such as (?<user>\w+) are listed by position only; the name is not displayed.

Tokens you will type most often

When the pattern is red

If the expression cannot be compiled, the match area is replaced by the browser's own SyntaxError text, something like Invalid regular expression: /(/g: Unterminated group. The wording differs between Chrome, Firefox and Safari, but the cause is the same: an unbalanced bracket, a quantifier with nothing before it (a pattern starting with a plus or a star), or a backslash followed by a character the engine does not recognise under the u flag. Fix the pattern and the results come back instantly.

A pattern that compiles but takes forever is a different problem. The tester runs synchronously in the page with no timeout, so a pattern prone to catastrophic backtracking, such as (a+)+$ against a long run of the letter a followed by a different character, can freeze the tab. Keep the test string short while you build a pattern with nested quantifiers.

Checking a pattern before it goes into production

• Form validation: paste a handful of valid and invalid inputs on separate lines, enable m, and anchor with ^ and $ so each line is tested on its own.
• Log mining: paste a few hundred log lines and watch the match count change as you tighten the pattern.
• Replace and split calls: the same expression object is used by String.prototype.replace and split, so what highlights here is exactly what those methods will act on.
• Analytics and firewall rules that accept JavaScript-style regex can be rehearsed here, but remember that tools built on PCRE, RE2 or POSIX differ in details such as lookbehind, possessive quantifiers and \A and \Z anchors.