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About this tool
Preview regex find-and-replace results with $1, $&, $<name> capture references and per-match breakdown before touching real data.
The Regex Replace Preview Tool shows the exact result of a regular-expression find and replace before you apply it to real data, expanding $1–$99, $&, $`, $', $<name> and $$ references per the ECMA-262 GetSubstitution algorithm that JavaScript's String.replace implements. Alongside the full output it lists every match with its position and what it becomes — the per-occurrence detail a one-shot replace never shows. Everything runs locally in the browser; no text leaves the page.
Open Regex Replace Preview Tool on AltFTool — it loads instantly in your browser.
Paste your code or data sample into the workspace.
Pick the format, conversion, or analysis you need.
Copy the polished result straight back into your project.
Every occurrence is listed with its index, matched text and expanded replacement — not just the final output.
Replacement references expand exactly as ECMA-262 specifies, including literal fallback for out-of-range groups.
Text is capped at 100,000 characters and matches at 5,000, so runaway zero-width patterns cannot hang the tab.
Wrap the part you want to keep in parentheses in the pattern, then reference it as $1, $2 and so on in the replacement — for example, find (\w+) (\w+) and replace with $2 $1 to swap two words. Named groups work too: (?<year>\d{4}) in the pattern becomes $<year> in the replacement.
$& inserts the entire matched text, letting you wrap or decorate matches without capture groups — replacing \d+ with [$&] turns 42 into [42]. Its siblings $` and $' insert everything before and after the match respectively, and $$ produces a literal dollar sign.
JavaScript tries the two-digit reference first: $10 means group 10 only if the pattern actually has 10 or more capture groups; otherwise it falls back to group 1 followed by a literal 0, and if even group 1 does not exist the whole token stays literal. This tool shows the expansion per match, which makes that behaviour visible immediately.
Not identically — the $1 syntax previewed here is JavaScript (also used by VS Code's search-and-replace), while sed and Perl use \1, and Python's re.sub uses \1 or \g<name>. The matching logic usually transfers, so preview the transformation here and translate only the replacement token syntax for your target tool.