What is Regex?

A regular expression (regex) is a sequence of characters that defines a search pattern. Think of it as a powerful "find" tool that can match not just exact text, but patterns of text — like "any email address" or "all phone numbers in this document."

Regex is used everywhere: form validation, search-and-replace in code editors, log file analysis, data extraction, and web scraping. Learning regex is one of the highest-leverage skills for developers, data analysts, and power users.

Basic Patterns

At its simplest, a regex is just literal text. The pattern hello matches the word "hello" anywhere in a string. But regex becomes powerful with special characters:

• . — matches any single character except newline. h.t matches "hat", "hit", "hot"
• \d — matches any digit (0-9). \d\d\d matches "123", "456"
• \w — matches any word character (letter, digit, underscore)
• \s — matches any whitespace (space, tab, newline)
• \D, \W, \S — the negated versions (non-digit, non-word, non-space)

Quantifiers — How Many?

Quantifiers specify how many of the preceding element to match:

• * — zero or more. ab*c matches "ac", "abc", "abbc", "abbbc"
• + — one or more. ab+c matches "abc", "abbc" but NOT "ac"
• ? — zero or one (optional). colou?r matches "color" and "colour"
• {3} — exactly 3. \d{3} matches exactly three digits
• {2,5} — between 2 and 5. \w{2,5} matches 2-5 word characters
• {3,} — 3 or more

Character Classes

Square brackets define a set of characters to match:

• [aeiou] — matches any vowel
• [a-z] — matches any lowercase letter
• [A-Za-z0-9] — matches any alphanumeric character
• [^0-9] — the caret negates: matches anything that is NOT a digit

Groups & Alternation

Parentheses create groups that can be captured and reused:

• (abc) — captures "abc" as a group
• (cat|dog) — alternation: matches "cat" OR "dog"
• (\d{3})-(\d{4}) — captures area code and number separately

Anchors — Where to Match

• ^ — start of string. ^Hello matches "Hello world" but not "Say Hello"
• $ — end of string. world$ matches "Hello world" but not "world class"
• \b — word boundary. \bcat\b matches "cat" but not "catalog"

Real-World Examples

Email Validation

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

Matches most standard email addresses like user@example.com.

Phone Numbers (US)

\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}

Matches: (555) 123-4567, 555-123-4567, 5551234567, 555.123.4567

URLs

https?:\/\/[\w.-]+\.[a-z]{2,}[\/\w.-]*

Matches most HTTP and HTTPS URLs.

IP Addresses

\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b

Dates (MM/DD/YYYY)

\d{2}\/\d{2}\/\d{4}

Practice Exercises

Try these in the 67Fresh Regex Tester:

1. Write a pattern to match all words starting with a capital letter
2. Match prices like $9.99, $100.00, $1,299.50
3. Extract all hashtags from a tweet (#word pattern)
4. Match hex color codes (#FFF or #FF00AA)
5. Validate a password: 8+ chars, at least one uppercase, one digit