Pattern Flags Input text

Highlighted output

Match list

Real input and output

Reordering a date with capture groups

Input

Pattern  (\d{4})-(\d{2})-(\d{2})
Replace  $3/$2/$1
Subject  2026-08-11

Output

11/08/2026

Greedy versus lazy quantifiers

Input

Subject  <b>one</b><b>two</b>
A        <b>.*</b>
B        <b>.*?</b>

Output

A  <b>one</b><b>two</b>  (matches everything)
B  <b>one</b>            (shortest match)

.* takes as much as possible, .*? as little as possible. This is where tag and quote matching diverges.

Errors you will hit

A dot swallows everything

. matches any character except a newline. Combined with * it runs past your intended stop. Switch to .*? or narrow the class, for example [^<]*.

Matching a literal special character

. + * ? ( ) [ ] { } | ^ $ \ are syntax characters. Prefix a backslash to match them literally — a literal period is \.

The same regex fails on the second call

In JavaScript a regex object with the g flag remembers lastIndex, so repeated test or exec calls resume from the previous position. Recreate the regex each time or reset lastIndex to 0.

Tools that pair well

Where a regex tester saves real time

Validate input rules

Test patterns for emails, slugs, employee IDs, or order references before you wire them into app validation logic.

Extract values from logs

Build patterns that pull UUIDs, request IDs, paths, or token-like strings out of noisy incident logs.

Preview replacements safely

Check what a pattern really matches before doing large-scale replacements or text cleanup in another tool or codebase.

Things that trip people up a lot

What do `g`, `i`, and `m` change?

g searches globally, i ignores case, and m changes how line anchors behave across multiple lines.

Why do regex syntax errors happen so often?

Missing escapes, unbalanced groups, or unsupported syntax are the common causes. Testing a smaller fragment first is usually the fastest fix.

Which regex engine does this page use?

It uses the browser JavaScript regex engine. That means behavior can differ from PCRE or some server runtimes, so production checks still matter.