Regex Tester: Regular Expression Cheat Sheet & Online Debugger

What Is a Regular Expression?

A regular expression (regex) is a sequence of characters that defines a search pattern. They're used for validating input, finding and replacing text, extracting data from strings, and parsing logs. Nearly every programming language has built-in regex support.

How to Test Regex Online

Use the tool.tl Regex Tester:

1. Go to tool.tl/regex-tester
2. Enter your pattern in the Regex field (e.g., \d+)
3. Enter test text in the input area
4. Matches highlight in real time
5. See capture groups and match details below

Regex Syntax Quick Reference

Character Classes

Pattern	Meaning	Example
.	Any character except newline	a.c → abc, a1c
\d	Digit [0-9]	\d+ → 123
\w	Word char [a-zA-Z0-9_]	\w+ → hello_world
\s	Whitespace (space, tab, newline)	\s+ → spaces
\D \W \S	Negations of above	Non-digit, non-word, non-space
[abc]	Character set: a or b or c	[aeiou] → vowels
[^abc]	Negated character set	[^0-9] → non-digit

Quantifiers

Quantifier	Meaning
*	0 or more
+	1 or more
?	0 or 1 (optional)
{n}	Exactly n times
{n,m}	Between n and m times
{n,}	n or more times

Anchors & Boundaries

Anchor	Meaning
^	Start of string (or line with multiline flag)
$	End of string (or line)
\b	Word boundary
\B	Non-word boundary

Ready-to-Use Regex Patterns

# Email address
^[\w.+-]+@[\w-]+\.[\w.]{2,}$

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

# IPv4 address
^(\d{1,3}\.){3}\d{1,3}$

# URL (http/https)
https?://[\w\-.]+(:\d+)?(/[\w\-./?%&=#]*)?$

# Date (YYYY-MM-DD)
^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$

# Hex color code
^#([0-9a-fA-F]{3}|[0-9a-fA-F]{6})$

# Positive integer
^[1-9]\d*$

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

# US ZIP code
^\d{5}(-\d{4})?$

# Credit card number (basic)
^\d{13,19}$

Capture Groups

Parentheses () create capture groups to extract specific parts of a match:

# Extract year, month, day from a date
Pattern: (\d{4})-(\d{2})-(\d{2})
Text:    2024-03-15
Group 1: 2024
Group 2: 03
Group 3: 15

// JavaScript
const m = '2024-03-15'.match(/(\d{4})-(\d{2})-(\d{2})/);
console.log(m[1]); // 2024
console.log(m[2]); // 03

# Python
import re
m = re.match(r'(\d{4})-(\d{2})-(\d{2})', '2024-03-15')
print(m.group(1))  # 2024

Frequently Asked Questions

Are regex patterns case-sensitive?

Yes, by default. Use the i flag to make matching case-insensitive: /hello/i matches Hello, HELLO, and hello. In the regex tester, you can toggle flags including i, g (global), and m (multiline).

How do I match a literal dot or asterisk?

Escape special characters with a backslash: \. matches a literal dot, \* matches a literal asterisk. Special characters that need escaping: . * + ? [ ] { } ( ) ^ $ | \

What's the difference between greedy and lazy matching?

Quantifiers are greedy by default (match as much as possible). Add ? after a quantifier to make it lazy (match as little as possible). On text <a><b>: <.*> matches the whole string, while <.*?> matches only <a>.