Regular Expressions

✅ What is a Regular Expression?

A regular expression (short: regex) is a sequence of characters that defines a search pattern, mainly used for string matching and manipulation (e.g., validation, extraction, replacement).

🔍 Basic Components of Regex

Literals
Match exact characters.
Example:
- cat matches "cat" in "concatenate".
Metacharacters
Special characters with specific meanings:
- . → Any single character except newline
- ^ → Start of string
- $ → End of string
- * → 0 or more repetitions
- + → 1 or more repetitions
- ? → 0 or 1 occurrence
- | → OR operator
- \ → Escape special characters

🔢 Character Classes

[abc] → Matches a, b, or c
[^abc] → Matches any character except a, b, or c
[a-z] → Matches lowercase letters
[0-9] → Matches digits

Predefined classes:

\d → Digit (0-9)
\D → Non-digit
\w → Word character (letters, digits, underscore)
\W → Non-word character
\s → Whitespace
\S → Non-whitespace

🔁 Quantifiers

a* → 0 or more a
a+ → 1 or more a
a? → 0 or 1 a
a{3} → Exactly 3 a
a{2,4} → Between 2 and 4 a
a{2,} → 2 or more a

🧩 Groups and Capturing

You can use groups if you need to access the information captured inside a group later, for example in back-references or from calling code.

(abc) → Capturing group
(?:abc) → Non-capturing group
(?P<name>abc) → Named capturing group
\1, \2 → Backreferences to captured groups

✅ Common Regex Patterns

Email Validation

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

Phone Number (US)
```
^$\d{3}$\s?\d{3}-\d{4}$
```

URL

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

IP Address (IPv4)

^((25[0-5]|2[0-4]\d|[01]?\d\d?)\.){3}(25[0-5]|2[0-4]\d|[01]?\d\d?)$

Date (YYYY-MM-DD)
```
^\d{4}-\d{2}-\d{2}$
```
Password (8+ chars, 1 uppercase, 1 digit)
```
^(?=.*[A-Z])(?=.*\d)[A-Za-z\d]{8,}$
```

⚡ Advanced Features

Lookahead / Lookbehind
- (?=...) → Positive lookahead
- (?!...) → Negative lookahead
- (?<=...) → Positive lookbehind
- (?<!...) → Negative lookbehind

Example:
(?=.*\d)(?=.*[A-Z]) ensures at least one digit and one uppercase letter.