Question 1

How do I reuse part of what I matched?

Accepted Answer

Wrap it in parentheses in the pattern, then refer to it in the replacement by number. Matching (\d{4})-(\d{2}) and replacing with $2/$1 turns 2026-08 into 08/2026. Groups are numbered by the position of their opening bracket, so nested groups count too.

Question 2

Is it $1 or \1 in the replacement?

Accepted Answer

In JavaScript, and therefore here, it is $1. The backslash form belongs to sed, Perl and Python's re.sub, and pasting it into JavaScript inserts a literal backslash and a one. This is the single most common reason a replacement copied from a Stack Overflow answer does nothing useful.

Question 3

Why is only the first occurrence being replaced?

Accepted Answer

The g flag is off. Without it a regex stops at the first match, so replace changes one occurrence and leaves the rest. It is the same flag that decides whether the tester finds one match or all of them.

Question 4

How do I put a literal dollar sign in the replacement?

Accepted Answer

Double it: $$ produces one dollar sign. A dollar followed by anything that is not a group reference is left alone, so a price like $50 usually survives untouched, but $1 in a price would be read as a group and disappear.

Question 5

Can I delete matches instead of replacing them?

Accepted Answer

Leave the replacement empty. Every match is removed and the text closes up around it, which is the quickest way to strip HTML tags, trailing whitespace or a repeated prefix from a list.

Question 6

Can I change the case of what I captured?

Accepted Answer

Not with a replacement string. JavaScript has no case operators like sed's \U and \L, and the only way is to pass a function to replace in real code. Here you can reorder, duplicate and surround captured text, but not recase it.

tooldura

Regex Find and Replace

About Regex Find and Replace

How to use Regex Find and Replace

Frequently Asked Questions