🔍 What are Regular Expressions?
Regular expressions (regex or regexp) are powerful patterns used to match, search, and manipulate text. They provide a concise and flexible way to identify strings of text, such as particular characters, words, or patterns. Regular expressions are supported in virtually every programming language and text editor, making them an essential tool for developers.
A regex pattern like \d{3}-\d{3}-\d{4} matches phone numbers in the format 555-123-4567. The pattern breaks down to: three digits (\d{3}), followed by a hyphen, three more digits, another hyphen, and four final digits. This concise notation replaces hundreds of lines of string parsing code.
Regex patterns consist of literal characters (which match themselves) and metacharacters (special symbols with special meanings). Metacharacters like ., *, +, ?, and [] give regex its power and flexibility, allowing you to express complex patterns succinctly.
🚀 How to Use This Tool
- Enter Your Pattern: Type your regex pattern in the pattern input field. Don't include the delimiters (/) - just the pattern itself.
- Add Test Text: Enter the text you want to test your pattern against in the test string field.
- Select Flags: Choose regex flags like global (g), case insensitive (i), or multiline (m) to modify pattern behavior.
- See Matches Highlighted: Matches are highlighted in real-time so you can see what your pattern captures.
- View Capture Groups: If your pattern uses capture groups (), see what each group matched.
- Debug and Refine: Adjust your pattern and see results instantly. Build complex patterns step-by-step.
🎯 Common Use Cases
✅ Input Validation
Validate user input in forms - emails, phone numbers, postal codes, credit cards, passwords. Ensure data matches expected formats before processing or storing in your application.
🔍 Text Search & Replace
Find and replace patterns in code, documents, or logs. Extract specific information from unstructured text data or reformat text to match new requirements efficiently.
📊 Log Parsing
Extract structured information from server logs, error messages, or API responses. Identify error patterns, extract timestamps, IP addresses, or user IDs from log files.
🌐 URL Routing
Match and extract parameters from URLs in web applications. Define dynamic routes like /users/:id using regex patterns for flexible URL handling and parameter extraction.
📝 Data Extraction
Scrape structured data from HTML, XML, or text documents. Extract emails, phone numbers, prices, dates, or any pattern-based information from unstructured content.
🛠️ Code Refactoring
Find and replace code patterns across your codebase. Rename variables, update function signatures, or modify code structure using powerful regex-based search and replace.
📐 Regex Syntax Reference
| Pattern | Description | Example |
|---|---|---|
. | Any character except newline | a.c matches "abc", "a5c" |
\d | Any digit (0-9) | \d{3} matches "123" |
\w | Word character (a-z, A-Z, 0-9, _) | \w+ matches "hello_123" |
\s | Whitespace (space, tab, newline) | \s+ matches multiple spaces |
* | 0 or more repetitions | a* matches "", "a", "aaaa" |
+ | 1 or more repetitions | a+ matches "a", "aaaa" |
? | 0 or 1 (optional) | colou?r matches color, colour |
{n} | Exactly n repetitions | \d{4} matches "2024" |
{n,m} | Between n and m repetitions | \d{2,4} matches "12", "123" |
^ | Start of string/line | ^Hello matches "Hello world" |
$ | End of string/line | end$ matches "The end" |
[abc] | Any character in set | [aeiou] matches vowels |
[^abc] | Any character NOT in set | [^0-9] matches non-digits |
| | OR operator | cat|dog matches "cat" or "dog" |
() | Capture group | (\d{3}) captures 3 digits |
🚩 Regex Flags Explained
g (global)
Find all matches in the string, not just the first one. Essential for search-and-replace operations or counting occurrences.
i (case insensitive)
Match letters regardless of case. /hello/i matches "Hello", "HELLO", "HeLLo".
m (multiline)
^ and $ match the start/end of each line, not just the entire string. Useful for processing multi-line text or log files.
s (dotall)
Makes . match newline characters too. By default, . matches everything except newlines.
u (unicode)
Enable full Unicode matching. Properly handle emoji, accented characters, and multi-byte Unicode characters.
y (sticky)
Match only at the exact position in the string (from lastIndex). Advanced feature for specific parsing scenarios.
💡 Common Patterns
Here are real-world regex patterns you can use and adapt:
Email
[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2, URL
https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b Phone (US)
\d{3}-\d{3}-\d{4} or \(\d{3}\)\s?\d{3}-\d{4} Date (YYYY-MM-DD)
\d{4}-\d{2}-\d{2} Credit Card
\d{4}[\s-]?\d{4}[\s-]?\d{4}[\s-]?\d{4} Hex Color
#[0-9A-Fa-f]{6} IP Address
\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b Username
^[a-zA-Z0-9_-]{3,16}$ 🔒 Privacy & Security
All regex testing happens entirely in your browser using native JavaScript regex engine. No server processing required - your patterns and test strings never leave your device. ToolZone itself does not track, save, or transmit any of your data.
💡 Pro Tips
Escape Special Characters
Metacharacters like ., *, +, ?, [, ] must be escaped with backslash when matching literally: \. matches a period.
Non-Greedy Matching
Add ? after quantifiers to make them non-greedy. .*? matches as few characters as possible instead of as many as possible.
Capture Groups
Use () to capture parts of matches for extraction. (\d{3})-(\d{3})-(\d{4}) captures area code, prefix, and line number separately.
Non-Capturing Groups
Use (?:) when you need grouping without capturing. (?:abc)+ matches "abcabc" without creating capture groups.
Lookaheads
(?=...) (positive lookahead) and (?!...) (negative lookahead) check what follows without consuming characters. \d(?=px) matches digits followed by "px".
Test Incrementally
Build complex patterns step-by-step. Start with a simple pattern, test it, then add complexity. This makes debugging much easier.
❓ Frequently Asked Questions
Why doesn't my pattern match? ▼
Common issues: forgetting to escape special characters (., *, etc.), incorrect quantifiers, missing flags (especially g for multiple matches or i for case insensitivity), or anchors (^, $) preventing partial matches. Use the tester to debug step-by-step.
What's the difference between .* and .*? ▼
.* is greedy - it matches as much as possible. .*? is non-greedy (lazy) - it matches as little as possible. For example, in "abc def", a.*f matches the entire string, but a.*?f would match "abc def" stopping at the first "f".
How do I match a literal backslash? ▼
In regex patterns, use \\ to match a single backslash. In many programming languages, you need to escape it again in string literals: "\\\\" in code becomes \\ in the regex engine, which matches one backslash.
Are regex patterns the same across all programming languages? ▼
Core syntax is similar, but there are differences. Some languages support features others don't (like lookbehinds, atomic groups). Flags may differ (some languages use inline modifiers like (?i)). Always test patterns in your target language environment.
What are capture groups and how do I use them? ▼
Capture groups use () to extract parts of matches. For example, (\d{'{3}'}})-(\d{'{3}'}})-(\d{'{4}'}} ) captures area code, prefix, and line number separately from a phone number. Use (?:) for non-capturing groups when you need grouping without extraction.
How do I test for multiple matches in the same string? ▼
Enable the global flag (g) to find all matches in your test string instead of just the first one. This is essential for search-and-replace operations or counting occurrences of a pattern.