Regex Tester

Test regular expressions with real-time match highlighting, capture groups, and replacement preview.

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Test String 0 chars
0 matches Enter a regex pattern and test string above.
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Regex Tester — Tips & Guide

Use this free online regex tester to write, debug and understand regular expressions instantly. See every match highlighted with its index and captured groups.

Use the g Flag for All Matches

Without the g (global) flag, the regex only finds the first match. Enable it to highlight and list every occurrence in your test string.

Test Edge Cases

Always test empty strings, strings with special characters, and very long inputs. Edge cases reveal hidden bugs in patterns that appear to work on simple examples.

Named Capture Groups

Use (?<name>...) to give groups readable names instead of numbered indices. Makes complex patterns far easier to maintain.

Cheat Sheet Patterns

Click any pattern in the cheat sheet below the editor to instantly load it into the regex input and see it applied to your test string.

A regular expression (regex) is a sequence of characters that defines a search pattern. It is used for string matching, validation, and find-and-replace operations. Regex is built into virtually every programming language and text editor, making it one of the most universally useful tools in development.
Enter your pattern in the regex field above and type or paste a test string in the textarea. Matches are highlighted in real time — no need to press any button. Toggle the g flag to find all occurrences, and use the match list to inspect each result and any captured groups.
^ (caret) matches the start of a string (or line in multiline mode). $ (dollar) matches the end of a string (or line). Together, ^pattern$ ensures the entire string matches the pattern exactly, with no extra characters before or after. This is essential for strict input validation like email or date formats.
The dot . matches any single character except a newline by default. Enable the s (dotAll) flag to make . match newlines too. To match any character including newlines without the s flag, use [\s\S]. To match any digit use \d, any word character \w, and any whitespace \s.
Developer Pro Tips

5 Things Experts Know

01
Validate JSON Before Parsing

Always validate JSON from external sources before passing to your parser. A single malformed response can crash an entire data pipeline if unhandled.

02
Use Named Capture Groups in Regex

Replace numbered groups like (\d+) with named groups (?P\d+). Your code becomes self-documenting and refactoring-safe.

03
Base64 is Encoding, Not Encryption

Base64 is trivially reversible. Never use it to "hide" sensitive data. It is only for safe transmission of binary data over text-based protocols.

04
SHA-256 vs MD5

Use SHA-256 for any integrity check that matters. MD5 is broken for security purposes — collisions can be generated in seconds on modern hardware.

05
UUIDs vs Sequential IDs

UUID v4 prevents enumeration attacks (users cannot guess adjacent IDs) and makes multi-database merges trivial. The storage cost is ~22 bytes vs 4 bytes for int — worth it.

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JSON Formatter →Base64 →Diff Checker →URL Encoder →Hash Generator →