Regex Basics for Beginners

What Is Regex?

Essential Building Blocks

• . (dot) — Matches any single character except newline. Example: c.t matches cat, cut, cot.
• \d — Matches any digit (0-9). \D matches any non-digit.
• \w — Matches any word character (letters, digits, underscore). \W matches non-word characters.
• \s — Matches any whitespace (space, tab, newline). \S matches non-whitespace.
• ^ — Matches the start of a string. $ matches the end.
• [ ] — Character class. [aeiou] matches any vowel. [0-9] matches any digit. [^abc] matches anything except a, b, or c.

Quantifiers — How Many?

• * — Zero or more. a* matches "", a, aa, aaa, etc.
• + — One or more. a+ matches a, aa, aaa but not "".
• ? — Zero or one (optional). colou?r matches both color and colour.
• {n} — Exactly n times. \d{4} matches exactly 4 digits.
• {n,m} — Between n and m times. \d{2,4} matches 2, 3, or 4 digits.
• {n,} — At least n times. \w{3,} matches words with 3+ characters.

Groups and Alternation

• ( ) — Capturing group. Groups parts of a pattern and captures the match for later use.
• (?: ) — Non-capturing group. Groups without capturing.
• | — Alternation (OR). cat|dog matches either cat or dog.
• Backreferences: \1 refers to the first captured group. (\w+) \1 matches repeated words like "the the".

10 Practical Regex Patterns

• Email (basic): [\w.-]+@[\w.-]+\.[a-zA-Z]{2,}
• US phone: \(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}
• URL: https?://[\w.-]+(?:/[\w./-]*)?
• IP address: \d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}
• Date (YYYY-MM-DD): \d{4}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[12]\d|3[01])
• Hex color: #[0-9a-fA-F]{3,6}
• HTML tag: <([a-z]+)[^>]*>.*?</\1>
• Whitespace trimming: ^\s+|\s+$ (use with replace to trim)
• Duplicate words: \b(\w+)\s+\1\b
• Password strength: ^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$

Common Mistakes to Avoid

• Greedy vs. lazy matching: .* is greedy (matches as much as possible). .*? is lazy (matches as little as possible). Use lazy matching when extracting content between delimiters.
• Forgetting to escape special characters: . matches ANY character. To match a literal dot, use \.
• Over-engineering: Don't use regex to parse HTML or validate complex formats like email addresses to RFC spec. Use a dedicated parser instead.
• Not anchoring: Without ^ and $, your pattern can match anywhere in the string. \d{5} matches any 5 consecutive digits, even inside longer strings.

Using Our Regex Tester Tool