Regular Expression Tester

Test and evaluate JavaScript regular expressions in real time. Highlight text matches, inspect captured subgroups, test replacement strings, and explore battle-tested patterns.

/ / gm
Flags:
0 matches found
Match Analysis & Groups: Matches breakdown

Regex Substitution / Replace

Substitution Output:

Mastering JavaScript Regular Expressions

Regular Expressions (often abbreviated as Regex or RegExp) are standardized sequences of characters that define specific search patterns. They are ubiquitous in software development for form data validation, string parsing, web scraping, and code refactoring.

ToolNexx's Regular Expression Tester evaluates your regex patterns natively inside your browser's JavaScript V8/SpiderMonkey engine. As you type, each match is discovered immediately, complete with positional character offsets and extracted capture groups ($1, $2).

Understanding Standard Regex Flags

  • g (Global): Searches for all possible matches throughout the entire string rather than stopping at the first match.
  • i (Case-Insensitive): Matches characters regardless of uppercase or lowercase letter distinctions.
  • m (Multiline): Causes the beginning (^) and end ($) anchors to match the start and end of every line rather than the whole string.
  • s (DotAll): Allows the wildcard dot (.) metacharacter to match newline characters (\n).
  • u (Unicode): Enables full Unicode code-point matching and strict handling of surrogate pairs.

Frequently Asked Questions

ToolNexx safeguards against runaway regex execution by setting a maximum match iteration cap (1,000 matches) and isolating evaluation in a guarded try/catch block.
Parentheses (...) in your pattern define capture groups. In the replacement field, $1 refers to the first group, $2 to the second, and $& inserts the entire matched string.