09. REGEX Cheat List

What is Regex?

Regular Expressions (regex) are sequences of characters that define search patterns. They are used for pattern matching and text processing.

Where Can Regex Be Used?

1. Programming & Scripting

Python (re module)
JavaScript (RegExp object)
Java (Pattern and Matcher classes)
C++ (<regex> library)
Go (regexp package)
Perl, PHP, Ruby, Rust, etc.

2. Command Line Tools

Grep → grep -E "pattern" file.txt
Sed → sed -E 's/old/new/g' file.txt
Awk → awk '/pattern/ {print $0}' file.txt

3. Text Editors & IDEs

VS Code, Sublime Text, Notepad++, Atom (Find & Replace with regex)
Vim/Neovim (:%s/pattern/replacement/g)

4. Databases

SQL (PostgreSQL, MySQL, MongoDB) → SELECT * FROM users WHERE email REGEXP '^[a-z0-9._%+-]+@[a-z0-9.-]+\\.[a-z]{2,}$'

5. Web Development

Form Validation (e.g., validating emails, phone numbers)
Search Engines (e.g., Google advanced search with regex)

6. Data Processing & Analytics

Log Parsing (Extracting specific patterns from logs)
Data Cleaning (Removing unwanted characters from text)

7. Cybersecurity & Ethical Hacking

Regex-based filtering (firewall rules, IDS/IPS systems)
Password strength validation

8. NLP (Natural Language Processing)

Tokenization (Splitting text into words/sentences)
Named Entity Recognition (NER)

Regex is powerful but can be complex. Let me know if you need practical examples for any specific use case.

1. Basic Syntax

Literals

abc → Matches exact string "abc"

Escape Characters

\. → Matches a literal .
\* → Matches a literal *
\\ → Matches a literal \
$ and $ → Matches literal ( and )

2. Character Classes

Common Character Classes

. → Any character (except newline)
\d → Any digit (0-9)
\D → Any non-digit
\w → Any word character (a-z, A-Z, 0-9, _)
\W → Any non-word character
\s → Any whitespace (space, tab, newline)
\S → Any non-whitespace

Custom Character Classes

[abc] → Matches a, b, or c
[^abc] → Matches anything except a, b, or c
[a-z] → Matches any lowercase letter
[A-Z] → Matches any uppercase letter
[0-9] → Matches any digit
[a-zA-Z0-9] → Matches any alphanumeric character

3. Quantifiers

* → 0 or more times (greedy)
+ → 1 or more times (greedy)
? → 0 or 1 time (optional)
{n} → Exactly n times
{n,} → At least n times
{n,m} → Between n and m times
*?, +?, ??, {n,m}? → Lazy versions (match as little as possible)

4. Position Anchors

^ → Start of a string
$ → End of a string
\b → Word boundary
\B → Not a word boundary

5. Grouping & Alternation

Grouping

(abc) → Capturing group
(?:abc) → Non-capturing group
(?P<name>abc) → Named capturing group

Alternation (OR)

cat|dog → Matches cat or dog

6. Lookaheads & Lookbehinds

Lookaheads

foo(?=bar) → Matches foo only if followed by bar
foo(?!bar) → Matches foo only if NOT followed by bar

Lookbehinds

(?<=abc)def → Matches def only if preceded by abc
(?<!abc)def → Matches def only if NOT preceded by abc

7. Wildcards & Special Tokens

. → Any character except newline
.* → Matches everything (greedy)
.*? → Matches everything (lazy)

8. Examples

Regex	Matches
`\d{3}-\d{2}-\d{4}`	SSN (`123-45-6789`)
`\b[A-Z][a-z]+\b`	Capitalized words
`^https?://\S+`	URLs
`\$\d+(\.\d{2})?`	Currency (`$10.99`)

9. Flags

i → Case-insensitive
g → Global (match all occurrences)
m → Multiline mode (^ and $ match per line)
s → Dotall (. matches newlines)
x → Ignore whitespace for readability