Mastering Regular Expressions 2026

"Regular expressions are difficult" - that's true. But once you learn them, you can reduce text processing time by 90%.

This guide covers the most commonly used regex patterns in practice with examples.

---

Regex Basics

Basic Syntax

Character Classes

[abc]     - One of a, b, c
[^abc]    - Except a, b, c
[a-z]     - a through z
[A-Z]     - A through Z
[0-9]     - 0 through 9
[a-zA-Z]  - All letters

Quantifiers

{n}       - Exactly n
{n,}      - n or more
{n,m}     - Between n and m
*         - 0 or more ({0,})
+         - 1 or more ({1,})
?         - 0 or 1 ({0,1})

Groups and Capturing

(abc)     - Capture group
(?:abc)   - Non-capturing group
(?<name>abc) - Named group
\1        - Backreference to first group

---

Practical Patterns: Validation

Email Address

// Basic pattern
const emailRegex = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;

// Test
emailRegex.test('user@example.com');  // true
emailRegex.test('invalid-email');      // false

Pattern Analysis:

^                     - Start
[a-zA-Z0-9._%+-]+     - Local part (1+ chars)
@                     - @ symbol
[a-zA-Z0-9.-]+        - Domain (1+ chars)
\.                    - . (escaped)
[a-zA-Z]{2,}          - TLD (2+ chars)
$                     - End

URL

const urlRegex = /^(https?:\/\/)?([\da-z.-]+)\.([a-z.]{2,6})([/\w .-]*)*\/?$/;

// Test
urlRegex.test('https://example.com');           // true
urlRegex.test('https://sub.example.com/path');  // true
urlRegex.test('invalid url');                   // false

Phone Number (US Format)

// US phone numbers
const usPhoneRegex = /^(\+1[-.\s]?)?\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}$/;

// Test
usPhoneRegex.test('(555) 123-4567');  // true
usPhoneRegex.test('555-123-4567');    // true
usPhoneRegex.test('+1 555 123 4567'); // true

Password Strength

// Minimum 8 chars, upper/lowercase, number, special char
const strongPassword = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$/;

// Pattern analysis
// (?=.*[a-z])   - At least 1 lowercase (lookahead)
// (?=.*[A-Z])   - At least 1 uppercase
// (?=.*\d)      - At least 1 digit
// (?=.*[@$!%*?&]) - At least 1 special char
// {8,}          - 8 or more characters

Credit Card Number

// Visa
const visaRegex = /^4[0-9]{12}(?:[0-9]{3})?$/;

// MasterCard
const mastercardRegex = /^5[1-5][0-9]{14}$/;

// All cards (with optional hyphens)
const cardRegex = /^\d{4}[- ]?\d{4}[- ]?\d{4}[- ]?\d{4}$/;

---

Practical Patterns: Text Extraction

HTML Tag Content Extraction

const html = '<div class="title">Hello World</div>';

// Extract tag content
const contentRegex = /<div[^>]*>(.*?)<\/div>/;
const match = html.match(contentRegex);
console.log(match[1]); // "Hello World"

// Remove all tags
const noTags = html.replace(/<[^>]*>/g, '');
console.log(noTags); // "Hello World"

Extract Domain from URL

const url = 'https://www.example.com/path/page.html';

const domainRegex = /^(?:https?:\/\/)?(?:www\.)?([^/]+)/;
const domain = url.match(domainRegex)[1];
console.log(domain); // "example.com"

Log File Parsing

const logLine = '[2026-02-21 14:30:45] ERROR: Connection timeout at module.js:42';

const logRegex = /\[(\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2})\] (\w+): (.+) at (.+):(\d+)/;

const [, datetime, level, message, file, line] = logLine.match(logRegex);

console.log({
  datetime,  // "2026-02-21 14:30:45"
  level,     // "ERROR"
  message,   // "Connection timeout"
  file,      // "module.js"
  line,      // "42"
});

CSV Parsing (Comma-separated)

const csvLine = 'John,"Doe, Jr.",30,"New York, NY"';

// Split by comma (ignore commas inside quotes)
const csvRegex = /(?:^|,)("(?:[^"]*(?:""[^"]*)*)"|[^,]*)/g;

const fields = [];
let match;
while ((match = csvRegex.exec(csvLine)) !== null) {
  fields.push(match[1].replace(/^"|"$/g, '').replace(/""/g, '"'));
}

console.log(fields); // ["John", "Doe, Jr.", "30", "New York, NY"]

---

Practical Patterns: Search and Replace

Phone Number Formatting

const phone = '5551234567';

// Add hyphens
const formatted = phone.replace(/(\d{3})(\d{3})(\d{4})/, '$1-$2-$3');
console.log(formatted); // "555-123-4567"

Number Formatting

const amount = '1234567890';

// Thousand separators
const formatted = amount.replace(/\B(?=(\d{3})+(?!\d))/g, ',');
console.log(formatted); // "1,234,567,890"

Masking

// Email masking
const email = 'username@example.com';
const masked = email.replace(/(.{2})(.*)(@.*)/, '$1***$3');
console.log(masked); // "us***@example.com"

// Phone number masking
const phone = '555-123-4567';
const maskedPhone = phone.replace(/(\d{3})-(\d{3})-(\d{4})/, '$1-***-$3');
console.log(maskedPhone); // "555-***-4567"

Whitespace Cleanup

const text = '  Hello    World  ';

// Trim leading/trailing whitespace
const trimmed = text.replace(/^\s+|\s+$/g, '');

// Normalize multiple spaces to one
const normalized = text.replace(/\s+/g, ' ').trim();
console.log(normalized); // "Hello World"

---

Advanced Features

Lookahead

// Positive Lookahead: (?=...)
// Match "foo" only if followed by "bar"
const regex = /foo(?=bar)/;
'foobar'.match(regex);  // ["foo"]
'foobaz'.match(regex);  // null

// Negative Lookahead: (?!...)
// Match "foo" only if NOT followed by "bar"
const regex2 = /foo(?!bar)/;
'foobaz'.match(regex2); // ["foo"]
'foobar'.match(regex2); // null

Lookbehind

// Positive Lookbehind: (?<=...)
// Numbers preceded by "$"
const priceRegex = /(?<=\$)\d+/g;
'$100 and $200'.match(priceRegex); // ["100", "200"]

// Negative Lookbehind: (?<!...)
// Numbers NOT preceded by "$"
const nonPriceRegex = /(?<!\$)\d+/g;
'$100 and 200'.match(nonPriceRegex); // ["200"]

Named Groups

const dateRegex = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/;
const match = '2026-02-21'.match(dateRegex);

console.log(match.groups.year);  // "2026"
console.log(match.groups.month); // "02"
console.log(match.groups.day);   // "21"

Non-Greedy

const html = '<div>Hello</div><div>World</div>';

// Greedy (default): Match as much as possible
/<div>.*<\/div>/.exec(html)[0];
// "<div>Hello</div><div>World</div>"

// Non-Greedy: Match as little as possible
/<div>.*?<\/div>/.exec(html)[0];
// "<div>Hello</div>"

---

Language-Specific Implementation

JavaScript

// Creation methods
const regex1 = /pattern/flags;
const regex2 = new RegExp('pattern', 'flags');

// Methods
regex.test(str);        // boolean
str.match(regex);       // array or null
str.replace(regex, replacement);
str.split(regex);
regex.exec(str);        // detailed match info

// Flags
// g: Global search
// i: Case insensitive
// m: Multiline
// s: dotAll (. matches newline)
// u: Unicode

Python

import re

# Compile
pattern = re.compile(r'pattern', re.IGNORECASE)

# Methods
re.match(pattern, string)   # Match from start
re.search(pattern, string)  # Search entire string
re.findall(pattern, string) # List all matches
re.sub(pattern, repl, string) # Replace

# Example
emails = re.findall(r'[\w.+-]+@[\w.-]+\.\w+', text)
cleaned = re.sub(r'\s+', ' ', text).strip()

Go

import "regexp"

// Compile
re := regexp.MustCompile(`\d+`)

// Methods
re.MatchString(s)          // bool
re.FindString(s)           // First match
re.FindAllString(s, -1)    // All matches
re.ReplaceAllString(s, r)  // Replace

// Example
numbers := re.FindAllString("a1b2c3", -1)
// ["1", "2", "3"]

---

Performance Optimization

Reuse Compiled Regex

// Slow: Compiles every time
function validate(email) {
  return /^[\w.+-]+@[\w.-]+\.\w+$/.test(email);
}

// Fast: Compile once
const emailRegex = /^[\w.+-]+@[\w.-]+\.\w+$/;
function validate(email) {
  return emailRegex.test(email);
}

Use Non-Capturing Groups

// Unnecessary capture
const regex = /(https?):\/\/(www\.)?(.+)/;

// Non-capturing for unneeded groups
const regex = /(?:https?):\/\/(?:www\.)?(.+)/;

Avoid Catastrophic Backtracking

// Dangerous: (a+)+ pattern
const bad = /^(a+)+$/;
'aaaaaaaaaaaaaaaaaaaaaa!'.match(bad); // Very slow

// Safe: Restructure the pattern
const good = /^a+$/;

---

Debugging Tips

Using Regex Tester

1. Enter pattern
2. Enter test string
3. Check highlighted matches
4. Verify capture groups

Step-by-Step Validation

// Validate complex patterns step by step
const parts = [
  '^',                    // Start
  '[a-zA-Z0-9._%+-]+',    // Local part
  '@',                    // @
  '[a-zA-Z0-9.-]+',       // Domain
  '\\.',                  // .
  '[a-zA-Z]{2,}',         // TLD
  '$',                    // End
];

const emailRegex = new RegExp(parts.join(''));

---

FAQ

Q1: What's the difference between . and \.?

A: . matches any character (wildcard), \. matches a literal dot.

Q2: What's the difference between * and +?

A: * matches 0 or more, + matches 1 or more.

• ab*c -> "ac", "abc", "abbc"
• ab+c -> "abc", "abbc" ("ac" doesn't match)

Q3: How do I ignore case?

A: Use the i flag.

• JavaScript: /pattern/i
• Python: re.IGNORECASE

Q4: How do I make . match newlines?

A: Use the s flag (dotAll).

• JavaScript: /pattern/s
• Python: re.DOTALL

Q5: Which characters need escaping?

A: \ ^ $ . | ? * + ( ) [ ] { } - escape these with \.

---

Conclusion

Regex essentials:

1. Basic syntax: ., *, +, ?, [], ()
2. Meta characters: \d, \w, \s, ^, $
3. Quantifiers: {n}, {n,}, {n,m}
4. Advanced features: Lookahead, Lookbehind, Named Groups

Practice is key. Test in real-time at Regex Tester.

---

Related Tools