Regular Expressions (Regex) Explained

What is Regex?

Regex (short for Regular Expression) is a sequence of characters that defines a search pattern. It is commonly used for searching, replacing, and validating text.

Why Use Regex?

Find text patterns (e.g., emails, phone numbers, dates).
Replace text (e.g., reformatting dates or removing special characters).
Validate input (e.g., checking if an email is correctly formatted).
Extract data (e.g., pulling out hashtags from a tweet).

Basic Regex Syntax

1. Literal Characters

Regular characters match themselves:

hello matches "hello" in a text.

2. Metacharacters (Special Characters)

. → Matches any character except a newline.
^ → Matches start of a string.
$ → Matches end of a string.
* → Matches 0 or more occurrences.
+ → Matches 1 or more occurrences.
? → Matches 0 or 1 occurrences.
{n} → Matches exactly n occurrences.

3. Character Classes (Character Sets)

[abc] → Matches a, b, or c.
[^abc] → Matches any character except a, b, or c.
[0-9] → Matches any digit (0-9).

4. Predefined Character Classes

\d → Matches any digit (0-9).
\D → Matches non-digit characters.
\w → Matches word characters (a-z, A-Z, 0-9, _).
\s → Matches whitespace (space, tab, newline).

5. Grouping and Capturing

(abc) → Groups abc together.
(?:abc) → Non-capturing group.

6. Alternation (OR Operator)

cat|dog → Matches "cat" OR "dog".

7. Lookarounds (Advanced)

(?=abc) → Positive lookahead.
(?!abc) → Negative lookahead.

Examples

1. Match an Email Address

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

2. Match a Phone Number (XXX-XXX-XXXX)

\d{3}-\d{3}-\d{4}

3. Match a Date (DD/MM/YYYY or DD-MM-YYYY)

\d{2}[/-]\d{2}[/-]\d{4}

Testing Regex

You can test regex patterns using:

Online tools like regex101.com.
Programming languages like Python (import re), JavaScript (RegExp), Java (Pattern).