Regex Tester Online — Test and Debug Regular Expressions in Real Time

Tools — views

What Is a Regular Expression?

A regular expression (regex) is a sequence of characters defining a search pattern. Supported by virtually every programming language, regex is used for text search, data validation, string extraction, and find-and-replace operations.

A simple example:

Pattern: ^\d{4}-\d{2}-\d{2}$
Meaning: Match a date in YYYY-MM-DD format
Test:    2024-05-19 ✓   2024-5-9 ✗   2024/05/19 ✗

Regex Syntax Quick Reference

SyntaxMeaningExample
.Any single character (except newline)a.c matches abc, a1c
*Previous element, 0 or more timesab*c matches ac, abc, abbc
+Previous element, 1 or more timesab+c matches abc, abbc — not ac
?Previous element, 0 or 1 timecolou?r matches color and colour
\dAny digit [0-9]\d{3} matches 3 digits
\wWord character [a-zA-Z0-9_]\w+ matches a word
\sWhitespace (space, tab, etc.)\s+ matches whitespace
^Start of string^Hello matches strings starting with Hello
$End of stringworld$ matches strings ending with world
[abc]Character class[aeiou] matches any vowel
()Capture group(\d+)-(\d+) captures two number segments

Common Regex Patterns

# Email validation
^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$

# US phone number
^(\+1)?[-.\s]?\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}$

# URL
https?://[^\s/$.?#].[^\s]*

# IP address
\b(?:\d{1,3}\.){3}\d{1,3}\b

# Strong password (8+ chars, upper, lower, digit)
^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$

# HTML tags
<[^>]+>

How to Test Regex Online

Use tool.tl's regex tester:

  1. Go to tool.tl/regex-tester
  2. Enter your regular expression in the pattern field
  3. Enter test text in the input area
  4. Matches are highlighted in real time, with capture group contents displayed

Regex Flags

  • i (case-insensitive): /hello/i matches Hello, HELLO, hello
  • g (global): find all matches, not just the first
  • m (multiline): ^ and $ match start/end of each line
  • s (dotAll): . matches newline characters too

Using Regex in Code

# Python
import re
pattern = r'\d+'
matches = re.findall(pattern, 'abc 123 def 456')
print(matches)  # ['123', '456']

# JavaScript
const regex = /\d+/g;
const matches = 'abc 123 def 456'.match(regex);
console.log(matches);  // ['123', '456']

# Replace
'Hello World'.replace(/World/, 'Regex')  // 'Hello Regex'

Frequently Asked Questions

Is regex syntax the same across all languages?

Core syntax is mostly consistent, but advanced features (lookaheads, Unicode support, named groups) vary between engines. The online tester uses a JavaScript engine — verify patterns in your target language before deploying to production.

Can regex hurt performance?

Complex patterns with nested quantifiers can cause catastrophic backtracking, making performance degrade exponentially on certain inputs. Always performance-test complex regexes before using in production code.

Is the tester free?

Yes — tool.tl's regex tester is completely free, with real-time match highlighting and capture group display. No account required.