Live Regex Tester & Debugger
Test regular expression patterns against test strings with live color-coded matches, capture groups, and flag options.
Complete Guide to Regular Expressions (Regex) & Syntax Mastery
A regular expression (regex or regexp) is a concise sequence of characters that specifies a search pattern in text. Used across programming languages (JavaScript, Python, PHP, Java, Golang, C#), regex powers text search algorithms, data sanitization, input form validation, lexical token parsing, and data scraping.
Regex Standard Flags
- g (Global): Finds all matching instances throughout the string rather than stopping at the first match.
- i (Case-Insensitive): Matches both uppercase and lowercase characters (e.g.,
[a-z]matchesA-Z). - m (Multi-line): Makes boundary anchors
^and$match the start and end of each individual line instead of the whole string. - s (dotAll): Allows the dot
.wildcard to also match newline\ncharacters.
Essential Character Classes
- \d : Matches any digit [0-9]
- \w : Matches word characters [a-zA-Z0-9_]
- \s : Matches whitespace (spaces, tabs, newlines)
- ^ / $ : Beginning / End of string or line
- + / * / ? : One or more / Zero or more / Optional (0 or 1)
- {min,max} : Exact quantifier range (e.g. {2,6})
Frequently Asked Questions (FAQs)
What is catastrophic backtracking in Regular Expressions?
Catastrophic backtracking (ReDoS - Regular Expression Denial of Service) occurs when nested quantifiers (such as (a+)+$) cause the regex engine's NFA state machine to evaluate exponential branch permutations on non-matching strings, freezing CPU threads. Prevent ReDoS by avoiding nested repetition operators and using atomic groups.
How do capturing groups vs non-capturing groups work?
Parentheses (pattern) create a capturing group, storing matched sub-strings into indexed references ($1, $2) or match arrays. Adding (?:pattern) creates a non-capturing group, grouping expressions for logical alternation without consuming memory allocations.
Are my regex queries executed safely in the browser?
Yes. All pattern evaluations run directly in your browser's native JavaScript regex engine using client-side processing without sending strings over external APIs.