Regex Tester & Debugger
Test, validate, and debug regular expressions in real-time. Features live match highlights, capture groups, replace mode, and a complete cheat sheet. All processing runs 100% client-side.
What is a Regular Expression (Regex) Tester?
Overview and core technical concepts
A Regex Tester evaluates JavaScript and PCRE pattern matches against sample target strings in real time, highlighting capture groups, matches, line offsets, and regex flag settings (g, i, m, s, u, y).
Why Test Regular Expressions?
Key advantages, developer speedups, and security benefits
Debug Complex Validation Logic
Ensure input validation regexes don't reject valid user strings or accept dangerous script injection strings.
Extract Structured Substrings
Test match capture groups before deploying pattern parsing in production code.
When Shouldn't You Use Regex?
Anti-patterns, limitations, and when to choose an alternative approach
HTML is non-regular grammar and cannot be parsed reliably with regex. Use dedicated DOM parsers or HTML query libraries.
Common Regex Pattern Examples
Sample inputs, expected outputs, and code patterns
Validating Email Addresses
Matches standard RFC email structure.
^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$Common Regex Mistakes
Frequent errors, security risks, and how to fix them
Frequently Asked Questions
A Regular Expression (commonly abbreviated as Regex or Regexp) is a sequence of characters that forms a search pattern. It is primarily used for pattern matching within text, character validation (such as checking if an email is formatted correctly), search-and-replace routines, and parsing structural data from text logs.
Flags modify the behavior of the regex search. 'g' (global) looks for all matches rather than stopping at the first one. 'i' (ignore case) performs case-insensitive matches. 'm' (multiline) matches caret (^) and dollar ($) at line breaks instead of just the whole string start/end. 's' (dotAll) makes dot (.) match newlines. 'u' enables Unicode code-point matching. 'y' performs sticky matching. 'd' (indices) yields character index offsets for all capture groups.
A capture group is defined by placing parentheses '( ... )' around part of a pattern. It stores whatever matched inside it for later reference (like $1). A named capture group is defined as '(?<name> ... )'. In addition to being indexed by number, it assigns a dictionary key to the match, allowing developer-friendly references like $<name> in replacements.
In regular expressions, you reference matched capture groups using the dollar symbol. Use '$1', '$2', etc., to insert numbered groups. For named capture groups, use '$<name>'. You can also use '$&' to insert the entire match text, '$`' for the text before the match, and "$'" for the text following the match.
No. ToolZeno operates 100% client-side in your local browser sandbox. The regular expressions run using your browser's native JavaScript RegExp engine. No input text, patterns, or files are ever sent to external servers, guaranteeing maximum data privacy for credentials, tokens, or private text layouts.
Catastrophic backtracking happens when a regular expression contains nested quantifiers (e.g., '(a+)+') that cause the engine to search an exponential number of matching paths when presented with slightly non-matching text. This can freeze the browser thread. ToolZeno prevents this by setting a strict watchdog timer (500ms limit) during execution, aborting the process if it runs too long.
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