What regex flavor does the tester use?

The tester uses JavaScript's built-in RegExp engine, which is ECMAScript-compliant. It supports lookaheads, lookbehinds (ES2018+), named capture groups, and Unicode mode, but not PCRE-specific syntax like \K or possessive quantifiers.

How do I test a regex with multiple flags?

Toggle the flag checkboxes in the tool interface. The most common flags are g (global — find all matches), i (case-insensitive), and m (multiline — makes ^ and $ match line boundaries).

What causes catastrophic backtracking in regex?

Patterns with nested quantifiers like (a+)+ applied to input that nearly-but-not-quite matches can cause exponential backtracking. Avoid ambiguous patterns where multiple paths through the regex can match the same characters.

Test & Debug Regex Patterns Online

Regular expressions are one of the most powerful text processing tools available, but writing and debugging them without immediate feedback is frustrating. A small mistake in a quantifier or character class can cause a pattern to match nothing, match too much, or perform catastrophically on certain inputs. devtoolkit.sh's Regex Tester provides a live editor where you type a pattern and test string simultaneously — matches are highlighted in real time, capture group values are shown in a table, and flags (global, case-insensitive, multiline, dotAll) can be toggled with checkboxes. The Find and Replace tool lets you apply your regex as a substitution and see the transformed text immediately, which is exactly what you need when writing a cleanup script or data normalization pipeline. If your goal is extracting specific patterns — email addresses, URLs, or phone numbers — from a body of text, the Extract Emails and Extract URLs tools offer purpose-built regex extraction without needing to write the pattern yourself.

.*Regex TesterHOT

Test regular expressions against text with real-time match highlighting.

F/RFind and Replace

Find text and replace it with options for case sensitivity and regex.

@Extract Emails

Extract all email addresses from a block of text.

FAQ

What regex flavor does the tester use?: The tester uses JavaScript's built-in RegExp engine, which is ECMAScript-compliant. It supports lookaheads, lookbehinds (ES2018+), named capture groups, and Unicode mode, but not PCRE-specific syntax like \K or possessive quantifiers.
How do I test a regex with multiple flags?: Toggle the flag checkboxes in the tool interface. The most common flags are g (global — find all matches), i (case-insensitive), and m (multiline — makes ^ and $ match line boundaries).
What causes catastrophic backtracking in regex?: Patterns with nested quantifiers like (a+)+ applied to input that nearly-but-not-quite matches can cause exponential backtracking. Avoid ambiguous patterns where multiple paths through the regex can match the same characters.