Regex Tester

Test and debug regular expressions with real-time pattern matching, group extraction, and detailed explanations.

Regex Pattern

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Regular Expression

//g

Flags

Test String

Match Results

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Common Regex Examples

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Key Features

Real-time pattern matching and highlighting

Capture group extraction and named groups

Pattern explanation and breakdown

Common regex examples library

Full flag support (global, case-insensitive, multiline)

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Understanding Regular Expressions

Pattern Matching: Regex finds patterns in text using special characters and quantifiers

Flags: Modifiers like 'g' (global), 'i' (case-insensitive), 'm' (multiline)

Groups: Parentheses capture parts of matches for extraction

Escaping: Use backslash (\) to match special characters literally

Tips & Best Practices

Start Simple: Build complex patterns step by step, testing each part

Use Raw Strings: In code, use raw strings to avoid double escaping

Avoid Greedy Matching: Use ? after quantifiers for non-greedy matching

Test Edge Cases: Empty strings, special characters, Unicode text

Performance: Avoid catastrophic backtracking with nested quantifiers

Documentation: Comment complex patterns and use named groups

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Frequently Asked Questions

What's the difference between global (g) and non-global matching?

Without the global flag, regex stops after the first match. With the global flag (g), it finds all matches in the text. This is essential for finding multiple occurrences.

How do capturing groups work?

Parentheses () create capturing groups that extract parts of the match. For example, in the pattern (\\d4)-(\\d2)-(\\d2), each set of parentheses captures year, month, and day separately.

Why isn't my regex working with multiline text?

By default, ^ and $ match the start/end of the entire string. Use the multiline flag (m) to make them match the start/end of each line within the string.

How can I make my regex case-insensitive?

Use the ignore case flag (i) to make your pattern match regardless of uppercase or lowercase letters. This is essential for user input validation.

Can I use regex for complex text processing?

Yes! Regex is powerful for data extraction, validation, and text transformation. However, for very complex parsing (like HTML/XML), consider dedicated parsers.

How do I escape special characters in regex?

Use a backslash (\) before special characters like . * + ? ^ $ [ ] ( ) | to match them literally. For example, \. matches a period.

What are lookaheads and lookbehinds?

Lookaheads (?=) and lookbehinds (?<=) are zero-width assertions that check for patterns without consuming characters. They're useful for complex matching conditions.

How do I match Unicode characters?

Use the Unicode flag (u) to enable proper Unicode matching. This allows patterns like \\p{L} for letters and \\p{N} for numbers across all languages.

Why is my regex slow or causing timeouts?

Catastrophic backtracking can occur with nested quantifiers like (a+)+. Use atomic groups, possessive quantifiers, or simplify your pattern to avoid this.