Regular Expression Tester

Real-Time Validation Tool

Test and debug regular expressions (Regex) in real-time directly in your browser with our free online tool. Instantly verify pattern matching, search & replace, and group captures to dramatically improve your development efficiency. Your input data is never stored on our servers, ensuring complete privacy even for sensitive information.

How to Use

Using the Regular Expression Tester is simple:

Step 1: Enter Your Regex Pattern
Enter the regular expression pattern you want to test in the top field. You can also select flags (g, i, m, s, u).
Step 2: Enter Test Text
Enter the text you want to match against your regex in the bottom textarea. Match results will be displayed in real-time.
Step 3: Review Match Results
Matched portions will be highlighted, and group capture details will be displayed. You can also see match count and index positions.

Your regex patterns and test text are never sent to our servers. All processing is done in your browser, ensuring complete privacy.

Regular Expression Pattern

Flags

g (global search)

i (case-insensitive)

m (multiline mode)

s (dot matches newlines)

u (Unicode)

Test Text

Practical Use Cases

Regular expressions are used in programming and text processing for:

Input Validation

Validate email addresses, phone numbers, postal codes, credit card numbers, and other formats using regex. Example: `/^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/` validates email addresses.

Data Extraction

Extract data matching specific patterns from log files or HTML. Examples include IP addresses, dates, URLs, and more from large volumes of text.

Search and Replace

Regex is used in text editors and IDEs to bulk replace strings matching specific patterns. Useful for variable refactoring, format changes, etc.

Parsing

Parse structured text like CSV, log files, and config files to extract necessary information.

Sanitization

Remove or replace dangerous characters and patterns from user input to reduce security risks. Helps prevent SQL injection and XSS attacks.

URL Routing

Web frameworks use regex to map URL patterns to route definitions and extract dynamic URL parameters.

What is a Regular Expression?

A regular expression (regex/regexp) is a powerful notation for representing string patterns. It's used to search, extract, or replace strings matching specific patterns.

Basic Regex Syntax

Regex uses special characters and metacharacters to describe patterns:

Literal: `abc` → Exact match "abc"

Dot `.`: Matches any single character

Asterisk `*`: 0 or more repetitions of preceding character

Plus `+`: 1 or more repetitions of preceding character

Question `?`: 0 or 1 occurrence of preceding character

Quantifier `{n,m}`: n to m occurrences of preceding character

Character class `[abc]`: Matches a, b, or c

Negation `[^abc]`: Matches any character except a, b, c

Start anchor `^`: Matches start of line

End anchor `$`: Matches end of line

Group `(abc)`: Grouping and capture

Alternation `a|b`: Matches a or b

Practical Regex Examples

Email: `^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$`
Phone (JP): `^0\d{1,4}-\d{1,4}-\d{4}$`
URL: `^https?://[\w\-.]+(:\d+)?(/[\w\-./?%&=]*)?$`
Date (YYYY-MM-DD): `^\d{4}-\d{2}-\d{2}$`
IPv4: `^((25[0-5]|2[0-4]\d|1\d{2}|[1-9]?\d)\.){3}(25[0-5]|2[0-4]\d|1\d{2}|[1-9]?\d)$`

Benefits of Using Regex

Improved Efficiency

Complex string operations can be accomplished in a single line of code. Dramatically reduces verbose code using loops and conditionals, improving readability and maintainability.

Flexible Pattern Matching

Handles not only fixed strings but also variable-length and conditional patterns. Complex conditions like "3 digits, hyphen, then 4 digits" can be expressed concisely.

Cross-Language Knowledge

Basic regex syntax is common across almost all programming languages (JavaScript, Python, Java, PHP, Ruby, etc.). Once learned, it can be applied to any language.

Reliable Validation

Strictly validate user input formats to reduce security risks and maintain data integrity.

Tips for Mastering Regex

Start Simple: Begin with simple patterns and gradually increase complexity.
Use a Tester: Regex can become complex. Use online testers like this to verify behavior while developing.
Escape Special Characters: To use special characters (`.`, `*`, `+`, `?`, `[`, `]`, `(`, `)`, `{`, `}`, `^`, `$`, `|`, `\`) as literals, escape them with backslash `\`.
Avoid Greedy Matching: `*` and `+` are greedy (longest match) by default. Use `*?` or `+?` for non-greedy matching when needed.
Use Groups: Grouping with `()` allows you to reference captured parts later. Especially useful for replacement operations.

Frequently Asked Questions

What's the difference between regex and wildcards?

Wildcards (`*`, `?`) are simple pattern matching used in shells and file searches. Regex is much more powerful, with complex conditions, quantifiers, grouping, backreferences, and other advanced features.

Is regex slow?

Well-written regex is very fast. However, overly complex patterns or patterns causing "catastrophic backtracking" can be slow. Keep patterns simple and efficient.

Is regex the same across all programming languages?

Basic syntax is common, but detailed specifications and available features vary by language and library. There are several flavors: PCRE (Perl-compatible), POSIX, ECMAScript (JavaScript), etc.

Can regex handle Japanese and Unicode?

Yes. Most modern regex engines support Unicode. In JavaScript, use the `u` flag to properly handle surrogate pairs and emoji.

Can I parse HTML or XML with regex?

Not recommended. HTML and XML have nested structures that are difficult to fully parse with regex alone. Use dedicated parser libraries (DOMParser, BeautifulSoup, lxml, etc.).

How can I improve regex performance?

1) Avoid unnecessary greedy matching, 2) Use specific patterns when possible, 3) Avoid unnecessary capture groups (use `(?:...)`), 4) Use anchors (`^`, `$`).

What's the best way to learn regex?

1) Learn basic metacharacters, 2) Try them in online testers like this, 3) Write practical patterns (email validation, etc.), 4) Read and understand existing patterns.

What regex engine does this tool use?

This tool uses JavaScript's standard regex engine (ECMAScript-compliant). Since it runs in the browser, behavior may differ slightly from server-side languages (Perl, Python, Java, etc.).