How to Replace Text with Regex

Problem

You need to replace multiple occurrences of a pattern in text, possibly reusing parts of the matched text in the replacement.

Prerequisites

• Understanding of regex patterns
• Knowledge of capture groups

Steps

1. 1. Define the search pattern

   Write a regex that matches the text you want to replace. Use capture groups for parts you want to reuse in the replacement. The g flag is needed to replace all occurrences; without it, only the first match is replaced.

   Input

   pattern: (\w+)@(\w+)

   Output

   captures name and domain

2. 2. Define the replacement

   The replacement can be a literal string or a function. Use $1, $2, etc. to reference capture groups in the replacement string. A function receives match details and can return dynamic replacements based on the matched content.

   Input

   replace with: $2.$1

   Output

   swaps name and domain

3. 3. Execute and verify

   Run the replacement and inspect the output. Check that all intended matches were replaced and that no unintended matches were affected. If the result is wrong, adjust the pattern or replacement and retry.

   Input

   [email protected] → example.alice

   Output

   verified

Expected result

Text with all matching patterns replaced according to the replacement specification.

Common errors

Only first occurrence replaced

Add the g (global) flag to replace all occurrences. Without g, replace only changes the first match.

Replacement includes literal $1 instead of captured text

Ensure the replacement string uses the correct syntax for capture group references. Different languages use $1 vs \1.

Troubleshooting

• If too many replacements occur, make the pattern more specific.
• Use a replacement function for complex logic that cannot be expressed as a simple string.

Security limits

• Regex replacement on untrusted input can be vulnerable to ReDoS. Use safe patterns and consider a timeout.