Regex Cheat Sheet

Every regex token I reach for, organized by job and annotated for engine support: PCRE2, JavaScript, Python, Go (RE2), Java, .NET, Ruby, and Rust. Anchors and boundaries, character classes, quantifiers, groups, lookarounds, flags, and the cross-engine differences that matter when you ship the same pattern across more than one runtime. Use the filter pills above the table to narrow rows by engine. Task-based walkthroughs (validate an email, match a date, parse a URL) are linked below the cheat sheet.

Here's a quick regular expressions cheat sheet with examples to get started:

1. Basic Characters:

   • .: Matches any character except newline. Example: a.c matches abc, adc.
   • \w: Matches a word character (letters, digits, _). Example: \w+ matches hello123.
   • \d: Matches any digit (0-9). Example: \d+ matches 123.
   • \s: Matches whitespace (space, tab, newline). Example: \s+ matches spaces in hello world.

2. Anchors:

   • ^: Matches the start of a string. Example: ^hello matches hello world.
   • $: Matches the end of a string. Example: world$ matches hello world.

3. Quantifiers:

   • *: Matches 0 or more occurrences. Example: a* matches aaa, a, or nothing.
   • +: Matches 1 or more occurrences. Example: a+ matches aaa, a, but not empty.
   • {n}: Matches exactly n occurrences. Example: a{3} matches aaa. Use {n,} for n or more, {n,m} for a range.

4. Groups:

   • (abc): Captures abc as a group.
   • (?:abc): Matches abc without capturing.
   • (?<name>abc): Captures abc and names it name.

Regular expressions (regex) are powerful tools for text matching and manipulation. This regex cheat sheet is a quick start regex tutorial, helping you understand regex patterns, regex syntax, and some practical applications. Whether you need a Python regex, Java regex, or JavaScript regex, this guide is a definite beginner must. Use the flavor filter to check compatibility and help you save time before pulling your hair out wondering why \w does not work in your Bash Script or why \p{Devanagari} is not working in your JavaScript Regex. I have also provided compatibility in table form later on in the page.

Regex Features and Examples

Character Classes

"John.Doe@techearl.com, 123-456-7890, 2024-01-15"

Anchors and Boundaries

"SuperHero saves the day! Not so super villain"

Quantifiers

Groups and Capturing

Lookaheads and Lookbehinds

Flags and Modifiers

PCRE Control Verbs

Control verbs are advanced features specific to PCRE (Perl-Compatible Regular Expressions). They allow you to manipulate the regex engine's backtracking and matching behavior directly, providing a level of control unavailable in most other regex engines. These verbs can optimize performance, enforce logic, or debug complex patterns by altering how the engine handles matches and failures.

Below is a brief introduction to each control verb and its function.

These verbs provide a high degree of control over how the regex engine processes and evaluates patterns, making them powerful tools for optimizing regex performance and logic in PCRE.

Advanced Examples

Validate an Email Address

^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$

Matches: user@example.com, hello.world@domain.org.

Match a Phone Number

^\+?\d{1,3}?[-.\s]?\(?\d{1,4}?\)?[-.\s]?\d{1,4}[-.\s]?\d{1,9}$

Matches: +1-800-555-5555, (800) 555-5555.

Extract URLs

https?:\/\/[a-zA-Z0-9\-\.]+\.[a-zA-Z]{2,}(\/\S*)?

Matches: https://example.com, http://domain.org/path.

Replace Multiple Spaces with One

\s{2,}

Replace with a single space to clean up text like "Hello world" into "Hello world".

Find Duplicate Words

\b(\w+)\s+\1\b

Matches: the the, hello hello.

Zero Punctuation and Regex in Action

Regex excels at handling edge cases, such as matching patterns without punctuation. For example, this regex ensures no punctuation in a string:

^[\w\s]+$

Matches: Hello World but not Hello, World!.

Tools for Testing and Debugging Regex

• Regex101: Interactive online regex tester.
• RegExr: Explore and test regular expressions visually.
• Regex Cheat Sheets: Downloadable PDFs for quick reference.

Step-by-step walkthroughs by task

The cheat sheet above is the syntax reference. For the patterns that come up over and over in real code, there is a focused walkthrough with multi-language examples (JavaScript, Python, PHP) for each.

Validation patterns

The character-class-and-anchors-heavy regexes you reach for in form validation.

• How to Match an Email Address with Regex: The practical pattern, the strict RFC 5321 pattern, runnable JavaScript, Python, and PHP code, plus the test table of common edge cases.
• How to Match a URL with Regex: Optional protocol, ports, query strings, fragments, and when to skip regex entirely and use the language's URL parser.
• How to Validate a Credit Card Number with Regex: Brand-detection patterns (Visa, MasterCard, Amex, Discover) plus the Luhn checksum that regex alone cannot compute.
• How to Validate a Strong Password with Regex: The stacked-lookahead pattern that enforces character-class requirements without caring about order.

Pattern matching for common data types

When you need to recognise a piece of structured text in the middle of unstructured input.

• How to Match an IPv4 and IPv6 Address with Regex: Octet-bounded IPv4 (rejects 999.999.999.999), full IPv6 with compression, CIDR notation, and the parser fallback.
• How to Match a Date with Regex (Multiple Formats): ISO 8601, US (MM/DD/YYYY), EU (DD/MM/YYYY), and what regex cannot do (leap years, month-length validation).
• How to Match Numbers with Regex: Integer, decimal, signed, scientific notation, thousands-separated, currency, and percent forms.
• How to Match a Hex Color Code with Regex: 3-digit and 6-digit forms, plus the 8-digit alpha-channel variant for modern CSS.
• How to Match HTML Tags with Regex (and why you probably shouldn't): When regex on HTML is acceptable, when it isn't, and the parser alternative.
• How to Match a Domain Name with Regex: RFC 1035 label rules, subdomain depth control, and the punycode form for internationalised domains. Pairs naturally with the DNS health check walkthrough for when you need to verify the domain actually resolves.

Features (zero-width assertions, groups)

The regex features that turn matching into context-aware extraction.

• How to Use Regex Lookaheads and Lookbehinds: Zero-width assertions for context, plus the password-validation use case and engine compatibility notes.
• How to Use Capturing Groups and Backreferences in Regex: Numbered groups, named groups, non-capturing groups, and how backreferences match the same text again.
• Regex Anchors: The position-asserting tokens like ^ and $, plus the multiline-mode flag.
• Regex Word Boundaries: Word-edge assertions including the Unicode-word edge cases that bite when matching non-ASCII text.
• How to Use Regex in .htaccess: Applying these patterns in Apache mod_rewrite for HTTPS redirects, www normalization, clean URLs, and access blocking.
• How to Use Regex in Nginx: The same patterns in Nginx location blocks and the rewrite directive, plus the location matching priority and why return beats rewrite.

Frequently asked questions

External regex references

For anything not on this page, the canonical online resources:

• Regex101: interactive tester with token-by-token explanation, supports PCRE, Python, JavaScript, Go, and .NET flavors
• MDN regex documentation: JavaScript regex reference
• Python re module: Python's regex documentation
• PCRE2 documentation: the C library PHP, R, and many tools use
• RegEx category on TechEarl: all the regex articles on this site