How do backreferences work in regex replace?

Use $1, $2 for numbered groups, $ for named groups, $& for the full match, $` for the text before the match, and $' for the text after. Example: 'John Smith'.replace(/(\w+) (\w+)/, '$2, $1') → 'Smith, John'.

How do I replace all occurrences with regex?

Use the g (global) flag: text.replace(/pattern/g, replacement). Without g, only the first match is replaced. Our tool applies the g flag by default in Replace mode.

Can I use a function as the replacement?

Yes in JavaScript code. The online tool uses a string replacement, but you can reference any group via $1–$9 or $ . For function-based replacements, copy the pattern to your code and use str.replace(re, (m, g1) => …).

What is the difference between replace and replaceAll?

String.replace() with a regex and the g flag replaces all matches. String.replaceAll() (ES2021) requires the g flag when used with a regex. Without a regex, replaceAll replaces all literal occurrences while replace only replaces the first.

Flowfiles ← Regex Tester

Regex Replace Online

Live replacement preview · Backreferences $1 $<name> · Flags g i m — free, no signup

Test regular expression substitutions online with a live preview of the result. Enter your pattern, write your replacement string with backreferences, and see the output update instantly — before copying it to your JavaScript, Python, or shell code.

Switch to Replace mode and preview your substitution in real time.

Open Replace Mode →

Replacement string reference

Pattern	Replaced by
`$&`	The full matched substring
`$1`, `$2`…	Numbered capture group (1-indexed)
`$<name>`	Named capture group
$`	Text before the match
`$'`	Text after the match
`$$`	Literal dollar sign

Common examples:

// Swap first and last name

'John Smith'.replace(/(\w+) (\w+)/, '$2, $1')

// → "Smith, John"

// Reformat date

'2024-03-15'.replace(/(?<y>\d{4})-(?<m>\d{2})-(?<d>\d{2})/, '$<d>/$<m>/$<y>')

// → "15/03/2024"

// Wrap matches in tags

text.replace(/\b\w{5,}\b/g, '<strong>$&</strong>')

Frequently asked questions

How do I replace all matches with a regex?

Enable the g (global) flag. Without g, String.replace() only replaces the first match. With g all occurrences are replaced. The tool applies g by default in Replace mode.

What is $& in a replacement string?

$& refers to the entire matched substring. For example, text.replace(/\w+/g, '[$&]') wraps every word in brackets.

How do case-insensitive replacements work?

Enable the i flag. The pattern will match regardless of case, but the replacement string is literal — the case of the original match is not preserved. To preserve case you need a function replacement in code.

Can I use multiline replace?

Enable the m flag to make ^ and $ match the start and end of each line. Combined with g, you can replace patterns at the start or end of every line in a multi-line string.

Related tools

Regex Tester Named Groups Regex Cheat Sheet Lookahead JSON ↔ CSV