Regex Patterns: Testing, Flags, and Capture Groups

• . — any character (except newline unless /s flag)
• \d, \w, \s — digit, word, whitespace shortcuts
• [a-z]+ — one or more lowercase letters
• (...) — capture group; $1 in replace strings
• ^ and $ — start and end anchors

Greedy quantifiers (`*`, `+`, `{n,m}`) match as much as possible; lazy (`*?`) match as little. Catastrophic backtracking on nested quantifiers can freeze browsers—test on realistic input sizes.

Capture groups let you extract parts of a match—`(\d{4})-(\d{2})-(\d{2})` on dates yields year, month, day as $1, $2, $3 in replace strings. Named groups (`(?<year>\d{4})`) improve readability in modern JavaScript.

Test replace before running bulk find-replace in an editor. A wrong `$1` reference can corrupt thousands of lines silently.

The Regex Tester on XSular Tools highlights matches in real time, lists capture groups, previews replace output, includes a library of common patterns, and debounces input so large test strings stay responsive—all in your browser.