Regular Expression Testing Tool Documentation

What is Regular Expression?

Regular expressions (regex) are powerful pattern-matching tools used in programming to search, extract, and manipulate text. They use special characters and syntax to define search patterns that can match strings of characters.

Regular expressions are widely used for data validation, text processing, log analysis, and search-and-replace operations. They provide a concise and flexible way to identify patterns in text.

How to Use This Tool

Choose a quick template or enter your regular expression pattern
Set appropriate flags (g for global, i for ignore case, m for multiline)
Input or load sample test text
View real-time match results and detailed information
Use the replace function to test substitutions

Regular Expression Flags

g

Global

Find all matches, not just the first one

i

Ignore Case

Case-insensitive matching

m

Multiline

^ and $ match line boundaries

Common Regular Expression Patterns

Basic Patterns

\\d - Any digit (0-9)

\\w - Word character (a-z, A-Z, 0-9, _)

\\s - Whitespace character

. - Any character except newline

Quantifiers

+ - One or more

* - Zero or more

? - Zero or one

{n} - Exactly n times

Anchors

^ - Start of line

$ - End of line

\\b - Word boundary

Practical Examples

Email Address Validation

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

Matches: one or more valid characters, followed by @, domain name, and 2+ letter extension.

Matches [email protected], [email protected]

Phone Number (US Format)

\$?([0-9]{3})\$?[-. ]?([0-9]{3})[-. ]?([0-9]{4})

Captures area code, exchange, and number with flexible formatting.

Matches (555) 123-4567, 555.123.4567, 555-123-4567

URL Matching

https?:\\/\\/(www\\.)?[-a-zA-Z0-9@:%._\\+~#=]{1,256}\\.[a-zA-Z0-9()]{1,6}\\b([-a-zA-Z0-9()@:%_\\+.~#?&//=]*)

Matches HTTP/HTTPS URLs with optional www prefix and various path formats.

Matches https://example.com, http://www.site.org/path

Strong Password Validation

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

Requires at least 8 characters with lowercase, uppercase, and digit.

Matches Password123, MyStr0ngP@ss

Advanced Features

Capturing Groups

Use parentheses () to capture parts of matches for replacement

([0-9]{4})-([0-9]{2})-([0-9]{2})

Lookahead/Lookbehind

Match based on what comes before/after without including it

(?=.*password)

Non-capturing Groups

Group without capturing using (?:pattern)

(?:http|https)://

Tips and Best Practices

Start simple and build complexity gradually
Use the global flag (g) to find all matches
Escape special characters with backslash (\)
Test thoroughly with various input examples
Use capturing groups for complex replacements
Consider performance with large texts

sed Command Generation

This tool automatically generates equivalent sed commands based on your regex pattern and replacement text, making it easy to use regex in Unix/Linux command line.

Search Mode (Empty Replacement)

When replacement text is empty, generates a search command to print matching lines only.

sed -n '/pattern/p' input.txt

Example: Pattern: error

Result: Prints all lines containing "error"

Replace Mode (With Replacement)

When replacement text is provided, generates a substitution command.

sed 's/pattern/replacement/flags' input.txt

Example: Pattern: foo, Replace: bar

Result: Replaces all "foo" with "bar"

Note: Special characters are automatically escaped. The generated command works in Unix/Linux/macOS terminals and Git Bash on Windows.

Learning Resources

Online Resources

• MDN Regular Expressions Guide

Regex Tester

Match Results

Match Details

Quick Templates

Replace Function