Regex Tester & Debugger — Test Regular Expressions Live
Test and debug regular expressions in real-time. Enter a pattern and test string to see matches, groups, and flags highlighted instantly.
Regex Tester
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The Formula
Regular expressions (regex) are patterns used to match character combinations in strings. A regex pattern is compiled into a finite state machine that scans the input string character by character, attempting to find matches according to the pattern rules. Flags modify matching behavior: g (global) finds all matches, i (case-insensitive) ignores case, and m (multiline) treats ^ and $ as line boundaries.
Variable Definitions
Regex Pattern
The regular expression pattern string defining the matching rules using special syntax like character classes, quantifiers, and groups.
Regex Flags
Modifiers that change matching behavior: g for global (find all matches), i for case-insensitive, m for multiline (^/$ match line boundaries).
Input Text
The string of text that the regex pattern is applied to for finding matches.
Matched Substring
A portion of the test string that satisfies the regex pattern. Multiple matches can be found with the global (g) flag.
Repetition Operator
Symbols like *, +, ?, and {n,m} that specify how many times a character or group should appear in the match.
How to Use This Calculator
- 1
Enter your regex pattern in the Pattern field (without delimiters).
- 2
Select optional flags: g for global matching, i for case-insensitive, m for multiline.
- 3
Enter or paste a test string to search against in the Test String field.
- 4
View match count, matched substrings, and whether a match was found.
Quick Reference
| From | To |
|---|---|
| . | Any character except newline |
| * / + / ? | Zero-or-more / One-or-more / Zero-or-one |
| \d / \w / \s | Digit / Word char / Whitespace |
Common Applications
- Validating input formats like email addresses, phone numbers, and URLs.
- Searching and replacing text patterns in code editors and data pipelines.
- Extracting specific data from unstructured text logs and documents.
- Parsing and tokenizing strings in compilers, interpreters, and data processors.
Regular expression matching pipeline — pattern compilation, string testing, and result output
Understanding the Concept
Regular expressions are a powerful tool for pattern matching and text manipulation that exist in virtually every programming language. The regex engine compiles a pattern into an internal finite automaton and processes the input string left to right. Patterns can include literal characters, metacharacters with special meaning (., *, +, ?, [], (), {}, ^, $), character classes (\d, \w, \s), and anchors (^ for start, $ for end). The global flag (g) is critical for finding all matches rather than just the first one. Case-insensitive flag (i) matches both uppercase and lowercase letters. Multiline flag (m) changes the behavior of ^ and $ anchors to match at every line boundary rather than just the string boundaries. Understanding greedy vs. lazy matching is also important: by default quantifiers are greedy (match as much as possible), adding ? makes them lazy (match as little as possible). Care must be taken with complex patterns as catastrophic backtracking can cause performance issues on certain inputs.
Frequently Asked Questions
Sources & References
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