What regex flavor does the tester use?

The tester uses JavaScript's built-in RegExp engine, which is compatible with most web and Node.js applications.

How do I match a literal dot or bracket?

Escape special characters with a backslash. Use \. for a literal dot, \[ for a bracket, and \\ for a literal backslash.

Can I use regex to validate complex formats?

Regex works well for pattern matching, but for complex validations like valid dates or email addresses, consider combining regex with application logic for complete accuracy.

How to Write Regex

4 min readDeveloper

Regular expressions are powerful but can be tricky to get right. This guide walks you through writing and testing regex patterns using a free browser tool that shows matches in real time and explains what each part of your pattern does.

Quick Steps

1
Open Regex Tester
Navigate to the Regex Tester on Toolin.
2
Enter your pattern
Type the regex pattern and set flags.
3
Add test text
Paste the text to match against.
4
Check matches
Review highlighted matches and adjust your pattern.

Regex Tester

Test and debug regular expressions

Open Tool

Step-by-Step: Write and Test a Regex

Open the Regex Tester tool

Navigate to the Regex Tester on Toolin.

Enter your pattern

Type your regular expression into the pattern field. Set flags like global (g), case-insensitive (i), or multiline (m) as needed.

Add test text

Paste or type the text you want to test against in the input area.

Review matches

Matches are highlighted in real time. Check that the pattern captures exactly what you intended.

Essential Regex Patterns

# Match an email address
[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}

# Match a phone number (US format)
\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}

# Match an IP address
\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b

# Match a date (YYYY-MM-DD)
\d{4}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[12]\d|3[01])

Tips for Writing Better Regex

Start simple and build up your pattern incrementally.
Use character classes like \d, \w, and \s instead of listing characters manually.
Test with edge cases including empty strings, special characters, and long inputs.
Use non-capturing groups (?:...) when you do not need to extract the match.
Anchor your patterns with ^ and $ when you need to match the full string.

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Frequently Asked Questions

What regex flavor does the tester use?: The tester uses JavaScript's built-in RegExp engine, which is compatible with most web and Node.js applications.
How do I match a literal dot or bracket?: Escape special characters with a backslash. Use \. for a literal dot, \[ for a bracket, and \\ for a literal backslash.
Can I use regex to validate complex formats?: Regex works well for pattern matching, but for complex validations like valid dates or email addresses, consider combining regex with application logic for complete accuracy.