RegEx Tester

A powerful tool for testing and visualizing regular expressions. Input your regular expression pattern, set flags, and see matches highlighted in real-time.

Regular Expression:

Enter your regular expression pattern

Flags:

RegEx flags

Test String:

Enter text to test against your regular expression

Result:

Matches found: 0

Typical Use Cases

Regular expressions are commonly used for pattern matching and validation in many programming scenarios. Web developers use RegEx for form validation to ensure that user inputs like email addresses, phone numbers, and passwords conform to specific formats. This helps improve data quality and security.

Regular expressions are also essential for text processing tasks like search and replace operations, data extraction, and parsing. They enable developers to efficiently match complex patterns in text, making them invaluable for tasks such as code linting, log analysis, and data scraping from unstructured sources.

In development workflows, RegEx is frequently used for search and replace operations in code, finding specific patterns in logs for debugging, and extracting structured data from unformatted text sources.

Regular Expression Basics

Pattern: The regex pattern defines what you're searching for
Flags: Modify how the pattern is applied
g - global (find all matches)
i - case-insensitive
m - multiline (^ and $ match start/end of lines)
s - dot matches newlines
u - unicode support

Character Classes

Character	Description
`\d`	Any digit (0-9)
`\w`	Any word character (a-z, A-Z, 0-9, _)
`\s`	Any whitespace character
`\D`	Any non-digit
`\W`	Any non-word character
`\S`	Any non-whitespace character
`[abc]`	Any character from the set (a, b, or c)
`[^abc]`	Any character not in the set
`[a-z]`	Any character in the range

Quantifiers and Anchors

Quantifiers

Character	Description
`*`	0 or more times
`+`	1 or more times
`?`	0 or 1 time
`{n}`	Exactly n times
`{n,}`	n or more times
`{n,m}`	Between n and m times

Anchors & Boundaries

Character	Description
`^`	Start of string/line
`$`	End of string/line
`\b`	Word boundary
`\B`	Non-word boundary

Common RegEx Patterns

Email:
^[\w.-]+@[\w.-]+\.[a-zA-Z]{2,}$
Validates email addresses with standard format: username@domain.tld
URL:
^(https?:\/\/)?([\da-z\.-]+)\.([a-z\.]{2,6})([\/*\w \.-]*)*\/?$
Matches web URLs with or without protocol, supports subdirectories
Date (YYYY-MM-DD):
^\d{4}-\d{2}-\d{2}$
ISO date format validation (doesn't check if date is valid)
Strong Password:
^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$
Requires minimum 8 characters with lowercase, uppercase, number, and special character
Positive Lookahead:
q(?=u)
Matches 'q' only if followed by 'u' (without including 'u' in the match)
Negative Lookahead:
q(?!u)
Matches 'q' only if NOT followed by 'u'

RegEx in Different Languages

JavaScript

// Using RegExp constructor
const regex = new RegExp('\d+', 'g');

// Using literal notation
const regex = /\d+/g;

// Test if string matches pattern
regex.test('123'); // returns true

// Find all matches
const matches = 'abc123def456'.match(regex);
// matches = ['123', '456']

// Replace using RegEx
'hello'.replace(/[aeiou]/g, '*');
// returns 'h*ll*'

Python

import re

# Check if pattern exists
if re.search(r'\d+', 'abc123'):
    print('Match found')

# Find all matches
matches = re.findall(r'\d+', 'abc123def456')
# matches = ['123', '456']

# Replace using RegEx
result = re.sub(r'[aeiou]', '*', 'hello')
# result = 'h*ll*'

# Using compile() for performance
pattern = re.compile(r'\d+')
matches = pattern.findall('abc123def456')

RegEx Best Practices

Avoid overuse:
Regular expressions can become unreadable quickly; use them only when appropriate
Test thoroughly:
Regular expressions can have unexpected edge cases and behavior
Use named capture groups:
Improves readability and easier to work with results
Comment complex patterns:
Break down complex expressions into smaller parts and document them
Beware of greediness:
Use non-greedy quantifiers (*?, +?) when appropriate
Performance considerations:
Avoid catastrophic backtracking with complex patterns on large inputs