Regex Tester & Debugger

Regular expressions (regex) are powerful pattern-matching tools that search, validate, and manipulate text using special syntax. This tester uses JavaScript's RegExp engine to execute your patterns against test strings in real-time, highlighting matches and providing detailed feedback.



The tool interprets regex syntax including character classes ([a-z]), quantifiers (* + ? {n,m}), anchors (^ $), groups (()), alternation (|), and special sequences (\d \w \s). When you enter a pattern and test text, the JavaScript engine compiles the regex and searches for matches, showing results instantly with visual highlighting.



Understanding regex requires learning metacharacters—characters with special meanings like . (any character), * (zero or more), + (one or more), and \ (escape). The tester helps visualize how patterns match text, making it easier to debug complex expressions and understand why patterns succeed or fail.

1. Data Validation
Validate user input like emails, phone numbers, URLs, or credit cards using regex patterns. Instead of writing complex string manipulation code, a single regex validates format: /^[\w.-]+@[\w.-]+\.\w+$/ matches email addresses. Test patterns against edge cases before deploying to production.



2. Text Search & Replace
Find and replace patterns in large text documents or codebases. Regex enables sophisticated find-replace operations: replace all dates in MM/DD/YYYY format with YYYY-MM-DD, extract all URLs from HTML, or remove duplicate whitespace. The tester helps craft accurate patterns before applying them.



3. Log File Analysis
Parse server logs, application logs, or analytics data by matching patterns. Extract timestamps, IP addresses, status codes, or error messages using regex. For example, /\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}/ matches IPv4 addresses. Test extraction patterns against sample logs before processing millions of lines.



4. Web Scraping & Data Extraction
Extract specific data from HTML, JSON, or structured text. While parsers are better for complex tasks, regex works well for simple extraction: pulling prices (/\$[\d,]+\.\d{2}/), phone numbers, or product codes from web pages. Test patterns against actual page content before scraping.



5. Code Linting & Analysis
Identify code patterns, style violations, or potential bugs using regex. Find TODO comments, deprecated function calls, or inconsistent naming conventions. Many code quality tools use regex internally to match problematic patterns in source code.

• Start simple, build complexity gradually: Begin with basic patterns and add complexity step-by-step. Test at each stage to ensure correctness. A working simple regex beats a broken complex one. Break complex patterns into smaller parts, test individually, then combine.



• Use anchors to control matching: ^ matches start of string, $ matches end. Without anchors, patterns match anywhere in text. /abc/ finds "abc" in "xabcy", but /^abc$/ matches only exactly "abc". Anchors prevent partial matches for validation.



• Escape special characters: Metacharacters (. * + ? [ ] { } ( ) ^ $ | \) have special meanings. To match them literally, escape with backslash: \. matches a period, \* matches an asterisk. Common mistake: forgetting to escape dots in email/URL patterns.



• Avoid greedy quantifiers when possible: * and + are greedy—they match as much as possible. This can cause unexpected results. Use non-greedy versions (*? +?) when you want minimal matches. Example: /<.*>/ matches entire "", but /<.*?>/ matches just "