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Free Regex Tester Online: Test, Debug, and Learn Regular Expressions

Code on screen showing regular expression patterns

Published: June 10, 2026•8 min read

Regular expressions (regex) are one of the most powerful tools in a developer's toolkit — and one of the most frustrating to debug. A single misplaced character can cause a pattern that looks correct to match nothing. Or worse, to match almost everything.

A regex tester lets you write your pattern and test it against real text instantly, with visual feedback on what matched and why. This guide covers the core syntax, the most useful patterns, and how to use a regex tester to debug expressions that are not working as expected.

Regex Syntax Reference

Most programming languages implement regex with the same core syntax. Here are the building blocks you need to know:

Pattern	Meaning	Example
.	Any character except newline	c.t matches cat, cot, cut
\d	Any digit (0–9)	\d+ matches 42, 100, 7
\w	Word character (a-z, A-Z, 0-9, _)	\w+ matches hello, user_1
\s	Whitespace (space, tab, newline)	\s+ matches spaces and tabs
^	Start of string	^Hello matches "Hello world"
$	End of string	world$ matches "Hello world"
*	0 or more of preceding	a* matches "", a, aa, aaa
+	1 or more of preceding	a+ matches a, aa, aaa (not "")
?	0 or 1 of preceding (optional)	colou?r matches color and colour
{n,m}	Between n and m repetitions	\d{2,4} matches 12, 123, 1234
[abc]	Any character in the set	[aeiou] matches any vowel
[^abc]	Any character NOT in the set	[^aeiou] matches consonants
(abc)	Capture group	(\d+) captures the matched digits
a\|b	a or b	cat\|dog matches cat or dog

5 Regex Patterns You Will Actually Use

Email address validation

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

Matches most valid email formats. Note: no regex can perfectly validate all RFC 5321 email addresses. Use this for basic format checking, not deliverability.

URL matching

https?:\/\/(www\.)?[\w\-\.]+\.[a-z]{2,}(\/[^\s]*)?

Matches http and https URLs, with optional www and path. Useful for extracting links from text.

Phone number (US)

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

Matches formats like (555) 123-4567, 555-123-4567, 5551234567, and +1 555 123 4567.

IP address (IPv4)

^((25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(25[0-5]|2[0-4]\d|[01]?\d\d?)$

Validates IPv4 addresses like 192.168.1.1. Each octet must be 0–255.

Strong password check

^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[^a-zA-Z\d]).{8,}$

Requires at least one lowercase letter, one uppercase letter, one digit, one special character, and minimum 8 characters. Uses lookaheads.

Why Your Regex Is Not Matching

The most common reasons a regex fails silently:

⚠️

Missing the global flag

By default, a regex only matches the first occurrence. Add the g flag to match all occurrences in the string.

⚠️

Anchors blocking partial matches

^ and $ anchor to string boundaries. If you add them to a pattern meant to find text inside a longer string, it will only match if the entire string is a match.

⚠️

Escaped characters not doubled

In most languages, the backslash must be escaped inside a string literal. \d in a JavaScript string becomes \\d. In a regex literal (/\d/), no escaping is needed.

⚠️

Case sensitivity

Regex is case-sensitive by default. The pattern hello does not match Hello. Add the i flag for case-insensitive matching: /hello/i.

⚠️

Greedy vs. lazy quantifiers

+ and * are greedy — they match as much as possible. Use +? or *? to match as little as possible. This matters when matching HTML tags, quoted strings, and delimited blocks.

How to Use the ToolzGo Regex Tester

1
Open the Regex Tester at toolzgo.com/tools/developer-tools/regex-tester
2
Enter your regular expression in the pattern field.
3
Paste the text you want to test in the input area.
4
Matches are highlighted in real time as you type.
5
Toggle flags (g, i, m) with the flag buttons to change matching behavior.

Frequently Asked Questions

Is regex the same across all programming languages?

The core syntax is largely the same, but there are differences in how flags work, which escape sequences are supported, and how edge cases like named groups are handled. JavaScript, Python, PHP, and Java all have their own regex engine with slight variations.

What does the "g" flag do?

The global flag (g) makes the regex match all occurrences in the string, not just the first one. Without it, methods like match() or exec() stop after the first match.

What is a capture group?

A capture group is a portion of the pattern wrapped in parentheses, like (\d+). The text matched by a capture group is stored separately and can be retrieved from the match result. Named groups use the syntax (?<name>\d+).

How do I match a literal dot or parenthesis?

Special characters like ., (, ), [, ], *, +, ? must be escaped with a backslash: \., $, $. Without escaping, they are interpreted as regex operators.

Can regex match across multiple lines?

By default, ^ and $ match the start and end of the entire string. Add the multiline flag (m) to make them match the start and end of each line instead. Use the dotall flag (s) to make . match newlines as well.

Related tools: JSON formatter, URL encoder, and the full regex tester.

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