# Regex Linter Detect error-prone and risky regex patterns, check for catastrophic backtracking, unanchored patterns, and provide rewrite suggestions > Canonical page: https://elysiatools.com/en/tools/regex-linter - **Category:** Development - **Keywords:** regex, linter, pattern, validation, performance, security, regular expression ## Overview ## Regex Linter ## Overview Regex Linter analyzes regular expressions for common anti-patterns, performance issues, and correctness problems. It helps you write better, safer, and more efficient regular expressions. ## Issues Detected ### Critical Issues (Performance) - **Catastrophic Backtracking**: Nested quantifiers like `(a+)+` that can cause exponential time complexity - **Quadratic Behavior**: Patterns like `(.*)*` that exhibit O(n²) performance - **Unbounded Repeats**: Greedy quantifiers without maximum length limits ### Error Issues (Correctness) - **Unescaped Dots**: Using `. ` when you mean \. (literal dot) - **Missing Anchors**: Patterns that can match anywhere in the text - **Invalid Character Classes**: Ranges like `[a-Z]` that include unexpected characters - **Octal Escapes**: Ambiguous `` escapes in modern regex ### Warning Issues (Readability) - **Observable Non-Greedy**: `.*?` patterns that could use negated character classes - **Redundant Escapes**: Unnecessary backslashes like `\-` - **Double Negation**: Confusing `[^[^]]` patterns - **Ambiguous Alternation**: `a|ab` where order matters ### Info Issues (Style) - **Non-Capturing Groups**: Suggesting `(?:...)` when groups aren't referenced - **Verbose Patterns**: Can simplify `ddd` to `d{3}` ## Check Levels - **basic**: Only critical performance issues - **standard**: Critical + errors + warnings (recommended) - **strict**: All issues including style suggestions ## Dialect Support - **JavaScript**: ES2018+ features including lookbehind and Unicode property escapes - **Python**: Recommends raw strings, supports verbose regex option - **PCRE**: Warns about backtracking verbs if used unintentionally - **Go (RE2)**: Warns about unsupported features (lookbehind, backreferences) - **Java**: Version-specific feature warnings ## Example Input: ```regex (.*+)a ``` Output: ```json { "summary": { "score": 50, "severity": "error", "issuesFound": 2 }, "issues": [ { "severity": "critical", "message": "Catastrophic backtracking risk", "suggestion": { "pattern": "[^a]*a", "explanation": "Use negated character class instead" } } ] } ``` ## Inputs - **Regex Pattern** (text): Enter regex pattern (e.g., ^(a+)+$)... - **Regex Flags** (text): g, i, m, s, u, y - **Target Dialect** (select) - **Check Level** (select) - **Include Rewrite Suggestions** (checkbox) ## When to use - When developing or debugging regular expressions to ensure they are efficient and error-free. - Before deploying regex in production to avoid performance bottlenecks and security risks. - When learning regex to understand best practices and avoid common pitfalls. ## How it works - Enter your regex pattern and optional flags in the input fields. - Select the target dialect (e.g., JavaScript, Python) and check level (basic, standard, strict). - The tool scans the pattern for issues such as catastrophic backtracking, unescaped characters, and missing anchors. - Receive a detailed report with severity levels, explanations, and rewrite suggestions if enabled. ## Use cases - Optimizing regex patterns in web applications to improve response times. - Validating user input patterns in form fields to prevent injection attacks. - Reviewing legacy code for regex performance issues during refactoring. ## Frequently asked questions ### What is catastrophic backtracking? It's a situation where nested quantifiers in a regex can cause exponential time complexity, leading to performance issues or denial-of-service. ### Which regex dialects are supported? The tool supports JavaScript, Python, PCRE, Go (RE2), and Java, with dialect-specific warnings. ### What does the check level affect? Basic checks only critical performance issues, standard includes errors and warnings, and strict adds style suggestions. ### Can I get suggestions for fixing issues? Yes, enable the 'Include Rewrite Suggestions' option to receive optimized pattern alternatives. ### Is this tool suitable for beginners? Absolutely, it helps identify common mistakes and learn best practices in regex writing. ## Related tools - [AI Regex Explainer](https://elysiatools.com/en/tools/ai-regex-explainer): Use AI to break down regular expressions into readable explanations - [Glob to Regex](https://elysiatools.com/en/tools/glob-to-regex): Convert file matching patterns (Glob) to regular expressions - [Named Group Tester](https://elysiatools.com/en/tools/named-group-tester): Parse and display named capture groups from regex patterns - [Regex Benchmark](https://elysiatools.com/en/tools/regex-benchmark): Compare performance of different regex patterns, identify bottlenecks, and detect degenerate cases - [Regex to String Generator](https://elysiatools.com/en/tools/regex-to-string-generator): Generate random strings that match a given regular expression pattern - [C# Regex Tester (.NET)](https://elysiatools.com/en/tools/csharp-regex-tester): Test .NET/C# regular expressions online with named groups ((?…)), IgnoreCase/Multiline/Singleline options, match highlighting, capture-group tables, a ready-to-paste C# snippet and an IgnorePatternWhitespace preview. - [Java Regex Tester](https://elysiatools.com/en/tools/java-regex-tester): Test java.util.regex patterns online: (?…) named groups, possessive quantifiers and atomic groups, Pattern flags, ASCII vs UNICODE\_CHARACTER\_CLASS \\d semantics, and ready-to-paste Java code. - [PCRE2 Regex Tester](https://elysiatools.com/en/tools/pcre-regex-tester): Test PCRE2 patterns online: every named-group syntax ((?P), (?), (\\u0027n\\u0027)), possessive quantifiers, atomic groups, \\A \\z \\h shorthands, plus pcre2grep and C API snippets. ## Samples - [Regex Pattern Alternatives](https://elysiatools.com/en/samples/regex-alternatives): Multiple ways to write the same regex pattern with different trade-offs in readability, performance, and accuracy - [Regex Replace Samples](https://elysiatools.com/en/samples/regex-replace): Collection of common and useful regex replacement patterns for text transformation and data cleaning - [Common Regex Patterns](https://elysiatools.com/en/samples/regex-samples): Commonly used regular expression patterns for validation and matching - [Risky Regex Patterns](https://elysiatools.com/en/samples/risky-regex): Collection of regex patterns that demonstrate security vulnerabilities, performance issues, and common anti-patterns to avoid ## Related content - [Regex Testing, Visualization, and Safety Workflow](https://elysiatools.com/en/hubs/regex-utility): Build reliable regular expressions by learning syntax, testing matches, visualizing structure, previewing replacements, benchmarking alternatives, and scanning for ReDoS risk.