Regex Patterns

Practical regular expression cookbook. Patterns for validation, parsing, extraction, and refactoring across JavaScript, Python, Go, and command-line tools.

When to Use

• - Validating user input (email, URL, IP, phone, dates)
• Parsing log lines or structured text
• Extracting data from strings (IDs, numbers, tokens)
• Search-and-replace in code (rename variables, update imports)
• Filtering lines in files or command output
• Debugging regexes that don't match as expected

Quick Reference

Metacharacters

Pattern	Matches	Example
INLINECODE0	Any character (except newline)	INLINECODE1 matches abc, INLINECODE3
INLINECODE4

Digit [0-9] | \d{3} matches 123 | | \w | Word char [a-zA-Z0-9_] | \w+ matches hello_123 | | \s | Whitespace [ \t\n\r\f] | \s+ matches spaces/tabs | | \b | Word boundary | \bcat\b matches cat not scatter | | ^ | Start of line | ^Error matches line starting with Error | | $ | End of line | \.js$ matches line ending with .js | | \D, \W, \S | Negated: non-digit, non-word, non-space | |

Quantifiers

Pattern	Meaning
INLINECODE26	0 or more (greedy)
INLINECODE27

1 or more (greedy) | | ? | 0 or 1 (optional) | | {3} | Exactly 3 | | {2,5} | Between 2 and 5 | | {3,} | 3 or more | | *?, +? | Lazy (match as few as possible) |

Groups and Alternation

Pattern	Meaning
INLINECODE34	Capture group
INLINECODE35

Non-capturing group | | (?P<name>abc) | Named group (Python) | | (?<name>abc) | Named group (JS/Go) | | a\|b | Alternation (a or b) | | [abc] | Character class (a, b, or c) | | [^abc] | Negated class (not a, b, or c) | | [a-z] | Range |

Lookahead and Lookbehind

Pattern	Meaning
INLINECODE42	Positive lookahead (followed by abc)
INLINECODE43

Negative lookahead (not followed by abc) | | (?<=abc) | Positive lookbehind (preceded by abc) | | (?<!abc) | Negative lookbehind (not preceded by abc) |

Validation Patterns

Email

CODEBLOCK0

URL

CODEBLOCK1

IP Addresses

CODEBLOCK2

Phone Numbers

CODEBLOCK3

Dates and Times

CODEBLOCK4

Passwords (Strength Check)

CODEBLOCK5

UUIDs

CODEBLOCK6

Semantic Version

CODEBLOCK7

Parsing Patterns

Log Lines

CODEBLOCK8

Code Patterns

CODEBLOCK9

Data Extraction

CODEBLOCK10

Language-Specific Usage

JavaScript

CODEBLOCK11

Python

CODEBLOCK12

Go

CODEBLOCK13

Command Line (grep/sed)

CODEBLOCK14

Search-and-Replace Patterns

Code Refactoring

CODEBLOCK15

Text Cleanup

CODEBLOCK16

Common Gotchas

Greedy vs lazy matching

CODEBLOCK17

Escaping special characters

CODEBLOCK18

Newlines and multiline

CODEBLOCK19

Backtracking and performance

CODEBLOCK20

Tips

• - Start simple and add complexity. \d+ is almost always enough — you rarely need [0-9]+.
• Test your regex on real data, not just the happy path. Edge cases (empty strings, special characters, Unicode) break naive patterns.
• Use non-capturing groups (?:...) when you don't need the captured value. It's slightly faster and cleaner.
• In JavaScript, always use the g flag for matchAll and global replace. Without it, only the first match is found/replaced.
• Go's regexp package uses RE2 (no lookahead/lookbehind). If you need those, use a different approach or the regexp2 package.
• INLINECODE54 (PCRE) is the most powerful command-line regex. Use it over grep -E when you need lookahead, \d, or \b.
• For complex patterns, use verbose mode (re.VERBOSE in Python, /x in Perl) with comments explaining each part.
• Regex is the wrong tool for parsing HTML, XML, or JSON. Use a proper parser. Regex works for extracting simple values from these formats, not for structural parsing.