Regex Tester: Frequently Asked Questions

The tester uses JavaScript's built-in RegExp engine, which supports most standard regex features: character classes, quantifiers, anchors, groups, backreferences, lookaheads, and lookbehinds. It does not support some PCRE-only features like atomic groups or possessive quantifiers.

Without anchors (^ and $), a pattern matches anywhere in the string. Add ^ at the start and $ at the end of your pattern to require it to match the entire string, not just a substring.

The g (global) flag makes the regex engine find all matches in the string rather than stopping after the first one. Without g, the tester shows only the first match.

Escape it with a backslash. To match a literal dot, use \. To match a literal plus, use \+. Special characters that need escaping: . * + ? ^ $ { } [ ] | ( ) \

.* matches zero or more of any character — it matches even an empty string. .+ requires at least one character. Use .+ when you need at least some content and .* when the matched part can be empty.

Enable the i flag in the tester. In code, add 'i' to the flags: new RegExp('pattern', 'gi') or /pattern/gi.

Regex can catch obviously invalid email addresses, but perfectly validating email per RFC 5322 requires an extremely complex pattern. A practical regex like ^[^@]+@[^@]+\.[^@]+$ catches most invalid addresses while accepting all valid ones.