Regex Tester

What is Regex Tester?

Regex Tester is a free online tool for testing and validating regular expressions. It provides real-time feedback with highlighted matches, detailed match information, and support for regex flags. Perfect for developers who need to build, test, and debug regex patterns.

Whether you are validating email addresses, extracting data, or searching text, this tool makes regex development quick and visual with instant match highlighting and detailed capture group information.

What is Regular Expression (Regex)?

Regular Expression (Regex or Regexp) is a sequence of characters that define a search pattern. It is used for pattern matching within strings, data validation, text search and replacement, and parsing.

Basic Regex Syntax

Common character classes:

• Dot: matches any single character
• \d: matches any digit (0-9)
• \w: matches any word character (letters, digits, underscore)
• \s: matches any whitespace

Quantifiers:

• Star: matches 0 or more times
• Plus: matches 1 or more times
• Question mark: matches 0 or 1 time

Example pattern for phone number:

\d\d\d-\d\d\d-\d\d\d\d
Matches: 123-456-7890

How to use Regex Tester?

Test a Regex Pattern

To test your regular expression:

1. Enter regex pattern in the pattern field
2. Select flags as needed (Global, Case insensitive, Multiline)
3. Enter test string in the test area
4. View results - matches are automatically highlighted in yellow
5. Check match details - see position and capture groups

Using Flags

Global (g) - Find all matches instead of just the first:

Pattern: \d+
Text: "I have 2 apples and 5 oranges"
With g: Matches "2" and "5"
Without g: Matches only "2"

Case Insensitive (i) - Ignore case when matching:

Pattern: hello
Text: "Hello HELLO hello"
With i: Matches all three
Without i: Matches only "hello"

Multiline (m) - ^ and $ match line starts/ends:

Pattern: ^test
Text: "test\ntest"
With m: Matches both lines
Without m: Matches only first "test"

Capture Groups

Use parentheses to capture parts of matches:

Pattern: (\w+)@(\w+)\.(\w+)
Text: "[email protected]"
Match: [email protected]
Groups: john, example, com

Features

• Real-time testing - Auto-test as you type
• Match highlighting - Visual feedback with yellow highlights
• Flag support - Global, case insensitive, multiline
• Match details - Position and count for each match
• Capture groups - View extracted groups from matches
• Error validation - Clear error messages for invalid patterns
• Monospace font - Clear display for code and patterns
• Auto-update - Results update automatically
• Match counter - See total number of matches

Use Cases

1. Email Validation

Validate email addresses:

Pattern:

\w+@\w+\.\w+

Test String:

[email protected]
[email protected]
invalid.email

2. Phone Number Extraction

Extract phone numbers:

Pattern:

\d\d\d-\d\d\d-\d\d\d\d

Test String:

Call me at 123-456-7890 or 555-123-4567

3. URL Detection

Find URLs in text:

Pattern:

https?://[\w\-\.]+\.\w+

Test String:

Visit https://example.com or http://test.org

4. Password Validation

Validate password strength - must have lowercase, uppercase, and digit with minimum 8 characters:

Pattern:

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

5. Date Format Extraction

Extract dates in YYYY-MM-DD format:

Pattern:

\d\d\d\d-\d\d-\d\d

Test String:

Important dates: 2025-12-10 and 2026-01-15

6. Word Boundary Matching

Match whole words only:

Pattern:

\bcat\b

Test String:

The cat sat on the catalog

Result: Matches "cat" but not "cat" in "catalog"

Common Regex Patterns

Email

^[\w\.-]+@[\w\.-]+\.\w+$

Phone (US format)

^\d\d\d-\d\d\d-\d\d\d\d$

URL

^https?://[\w\-\.]+\.\w{2,}(/.*)?$

Date (YYYY-MM-DD)

^\d\d\d\d-\d\d-\d\d$

Hex Color

^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$

Username (alphanumeric + underscore, 3-16 chars)

^[a-zA-Z0-9_]{3,16}$

Tips for Best Results

1. Start simple: Begin with basic patterns and add complexity gradually

2. Use the global flag: Enable it to find all matches, not just the first one

3. Test edge cases: Try empty strings, special characters, and unexpected input

4. Escape special characters: Use backslash before special regex characters

5. Be specific: Use anchors (^ and $) to match exact patterns

6. Use capture groups: Extract specific parts of matches with parentheses

Common Regex Mistakes

1. Forgetting to Escape Special Characters

Wrong: Use dot directly (matches any character)
Right: Use backslash-dot (matches literal period)

2. Not Using Word Boundaries

Wrong: Pattern "cat" matches "cat" in "catalog"
Right: Pattern "\bcat\b" matches only whole word "cat"

3. Greedy vs Lazy Matching

Greedy matching takes as much as possible.
Lazy matching (add question mark) takes as little as possible.

Regex in Different Languages

JavaScript

const regex = /\d+/g;
const text = "I have 2 apples and 5 oranges";
const matches = text.match(regex);
// Result: ["2", "5"]

Python

import re
pattern = r'\d+'
text = "I have 2 apples and 5 oranges"
matches = re.findall(pattern, text)
# Result: ['2', '5']

Java

Pattern pattern = Pattern.compile("\\d+");
Matcher matcher = pattern.matcher("I have 2 apples");
while (matcher.find()) {
    System.out.println(matcher.group());
}

Frequently Asked Questions

Q: What does the "g" flag do?

A: The global flag (g) makes the regex find all matches in the text instead of stopping after the first match.

Q: How do I match a literal dot or other special character?

A: Use a backslash to escape special characters. For example, backslash-dot matches a literal period.

Q: Why is not my pattern matching?

A: Common issues include forgot to escape special characters, case sensitivity (try the i flag), or pattern is too specific or too general.

Q: What is the difference between star and plus?

A: Star matches 0 or more occurrences (can match empty string), while plus matches 1 or more occurrences (requires at least one character).

Q: How do I make matching case-insensitive?

A: Enable the "i" flag (case insensitive). This makes the pattern match both uppercase and lowercase letters.

Privacy & Security

Your privacy is important to us:

• No data is sent to any server
• All testing happens in your browser
• No cookies or tracking
• No account or login required
• Completely free to use

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