Regular Expressions

Regular expressions are pattern-matching sequences used to search, extract, validate, and transform text. This reference covers regex from fundamentals to advanced techniques, with emphasis on practical infrastructure applications.

Why Regex Matters

Regex appears in virtually every technical domain:

Domain	Applications
Log Analysis	Extract timestamps, IPs, error codes, usernames from logs
Configuration Management	Validate syntax, find misconfiguration, audit settings
Security	IOC extraction, threat hunting, SIEM queries, compliance audits
Network Engineering	Parse configs, extract IPs/MACs/VLANs, validate addressing
Automation	Data extraction in scripts, API response parsing, file processing
Development	Input validation, search/replace, code refactoring
Data Processing	ETL pipelines, data cleaning, format conversion

Domain

Applications

Log Analysis

Extract timestamps, IPs, error codes, usernames from logs

Configuration Management

Validate syntax, find misconfiguration, audit settings

Security

IOC extraction, threat hunting, SIEM queries, compliance audits

Network Engineering

Parse configs, extract IPs/MACs/VLANs, validate addressing

Automation

Data extraction in scripts, API response parsing, file processing

Development

Input validation, search/replace, code refactoring

Data Processing

ETL pipelines, data cleaning, format conversion

Curriculum

Module	Description	Status
1. Fundamentals	Literal characters, metacharacters, escaping, basic matching	Core
2. Character Classes	Sets, ranges, negation, shorthand classes (\d, \w, \s)	Core
3. Quantifiers	Repetition operators, greedy vs lazy matching, possessive quantifiers	Core
4. Anchors & Boundaries	Position matching, word boundaries, line/string anchors	Core
5. Groups & Capturing	Capturing groups, backreferences, non-capturing groups, named groups	Intermediate
6. Alternation & Conditionals	OR logic, conditional patterns, branch reset	Intermediate
7. Lookahead & Lookbehind	Zero-width assertions, positive/negative lookaround	Advanced
8. Regex Flavors	BRE, ERE, PCRE, JavaScript, Python, Vim differences	Reference
9. Infrastructure Patterns	Production-ready patterns for IPs, MACs, logs, configs	Reference
10. Tool Integration	grep, sed, awk, ripgrep, Python, JavaScript usage	Reference

Module

Description

Status

1. Fundamentals

Literal characters, metacharacters, escaping, basic matching

Core

2. Character Classes

Sets, ranges, negation, shorthand classes (\d, \w, \s)

Core

3. Quantifiers

Repetition operators, greedy vs lazy matching, possessive quantifiers

Core

4. Anchors & Boundaries

Position matching, word boundaries, line/string anchors

Core

5. Groups & Capturing

Capturing groups, backreferences, non-capturing groups, named groups

Intermediate

6. Alternation & Conditionals

OR logic, conditional patterns, branch reset

Intermediate

7. Lookahead & Lookbehind

Zero-width assertions, positive/negative lookaround

Advanced

8. Regex Flavors

BRE, ERE, PCRE, JavaScript, Python, Vim differences

Reference

9. Infrastructure Patterns

Production-ready patterns for IPs, MACs, logs, configs

Reference

10. Tool Integration

grep, sed, awk, ripgrep, Python, JavaScript usage

Reference

Quick Reference

Essential Metacharacters

Character Meaning Example

Character	Meaning	Example
`.`	Any character except newline	`a.c` matches "abc", "aXc"
`\`	Escape metacharacter	`\.` matches literal "."
`^`	Start of line	`^Error` matches "Error" at line start
`$`	End of line	`\.log$` matches lines ending in ".log"
`\|`	Alternation (OR)	`cat\|dog` matches "cat" or "dog"
`()`	Grouping	`(ab)+` matches "ab", "abab"
`[]`	Character class	`[aeiou]` matches any vowel

.

Any character except newline

a.c matches "abc", "aXc"

\

Escape metacharacter

\. matches literal "."

^

Start of line

^Error matches "Error" at line start

$

End of line

\.log$ matches lines ending in ".log"

|

Alternation (OR)

cat|dog matches "cat" or "dog"

()

Grouping

(ab)+ matches "ab", "abab"

[]

Character class

[aeiou] matches any vowel

Quantifiers Summary

Greedy Lazy Meaning

Greedy	Lazy	Meaning
`*`	`*?`	Zero or more
`+`	`+?`	One or more
`?`	`??`	Zero or one
`{n}`	`{n}?`	Exactly n
`\{n,}`	`\{n,}?`	n or more
`\{n,m}`	`\{n,m}?`	Between n and m

*

*?

Zero or more

+

+?

One or more

?

??

Zero or one

{n}

{n}?

Exactly n

\{n,}

\{n,}?

n or more

\{n,m}

\{n,m}?

Between n and m

Shorthand Classes

Shorthand Equivalent Meaning

Shorthand	Equivalent	Meaning
`\d`	`[0-9]`	Digit
`\D`	`[^0-9]`	Non-digit
`\w`	`[A-Za-z0-9_]`	Word character
`\W`	`[^A-Za-z0-9_]`	Non-word character
`\s`	`[ \t\n\r\f]`	Whitespace
`\S`	`[^ \t\n\r\f]`	Non-whitespace

\d

[0-9]

Digit

\D

[^0-9]

Non-digit

\w

[A-Za-z0-9_]

Word character

\W

[^A-Za-z0-9_]

Non-word character