Regex Cheat Sheet

This regex cheat sheet is a single-page reference for every regular expression token, grouped by job: character classes, anchors, quantifiers, groups, lookarounds, flags, and Unicode properties. Each row is annotated for engine support across PCRE2 (PHP), JavaScript, Python, Go, Java, .NET, Ruby, Rust, and POSIX, so you can copy a pattern and know whether it runs in your runtime.

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. The filter pills above the table narrow rows by engine. Task-based walkthroughs (validate an email, match a date, parse a URL) are linked below.

Regex cheat sheet with examples

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.

Extended regex (ERE) cheat sheet: grep -E, sed -E, and awk

Most of this page documents PCRE / Perl-style regex, which is what JavaScript, Python, PHP, and the rest of the engines above use. The POSIX command-line tools are a different world, and this is the part that trips people up when they move a pattern from a language into the shell. POSIX defines two flavors, and the one you want for grep -E / egrep, sed -E (sed -r on GNU), and awk is Extended Regular Expressions (ERE). The other flavor, Basic Regular Expressions (BRE), is the default for plain grep, sed without -E, and expr.

The crux of the difference is what counts as a metacharacter. In ERE, + ? | ( ) { } are special on their own. In BRE, those same characters are literal until you escape them, so you have to write \+ \? \| \( \) \{ \} to get the metacharacter meaning. That inversion is why the exact same pattern behaves differently between grep and grep -E.

ERE vs BRE: the same operators, escaped differently

"colour color colours"

A quick way to remember it: *, ., ^, $, and [...] mean the same thing in both flavors. It is only + ? | ( ) { } that flip, and ERE is the flavor where they work without the backslash.

ERE has no \d, \w, \b: use POSIX classes instead

The shorthand classes everyone reaches for, \d for a digit, \w for a word character, \b for a word boundary, are Perl/PCRE extensions, not part of POSIX ERE. GNU grep/sed quietly support \w and \b as GNU extensions, but awk and any portable script will not, and \d is not standard ERE at all. The portable answer is the POSIX bracket-expression classes, which must sit inside an outer [...], so a digit is [[:digit:]], not [:digit:] on its own.

There is no POSIX class for a word boundary. GNU tools offer \b (and the GNU-specific \< / \> for the start and end of a word), but awk has none, so in portable scripts you anchor on [[:space:]] or the start/end of the line instead.

So the same "find a 3-digit number" pattern looks like this across the tools. Note [[:digit:]] carries the doubled brackets and the interval needs the unescaped braces that ERE gives you:

# ERE: grep -E, egrep, awk, sed -E
grep -E '[[:digit:]]{3}' file.txt

# BRE: plain grep needs the braces escaped
grep '[[:digit:]]\{3\}' file.txt

# awk uses ERE; print lines with a 3-digit run
awk '/[[:digit:]]{3}/' file.txt

The full POSIX class list ([:upper:], [:lower:], [:punct:], [:xdigit:], and the rest) is in the POSIX Character Classes block at the top of this cheat sheet, with engine support flagged per class.

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

Recommended books

This page is the quick reference. When you want the full mental model behind the syntax, these are the canonical regex books:

• Mastering Regular Expressions (Jeffrey Friedl, 3rd edition). The definitive deep-dive on how regex engines actually work: backtracking, NFA versus DFA, and the optimisation that makes a pattern fast or catastrophic. Dense, and unmatched once you are past the basics.
• Regular Expressions Cookbook (Jan Goyvaerts and Steven Levithan, 2nd edition). Problem-then-solution recipes across eight languages (JavaScript, Python, PHP, Java, .NET, Ruby, Perl, VB). The one to keep next to the keyboard.
• Learning Regular Expressions (Ben Forta). The gentlest on-ramp: short, current, and example-driven. A good first book if you are still finding your feet.