Advanced Regex
Once you've mastered the basics, these advanced features unlock even more power.
Lookahead
Match only if followed by pattern (doesn't consume):
// Positive lookahead (?=)
/foo(?=bar)/ // "foo" only if followed by "bar"
"foobar".match(/foo(?=bar)/) // ["foo"]
// Negative lookahead (?!)
/foo(?!bar)/ // "foo" only if NOT followed by "bar"
"foobaz".match(/foo(?!bar)/) // ["foo"]
Lookbehind
Match only if preceded by pattern:
// Positive lookbehind (?<=)
/(?<=@)\w+/ // Word after @
"user@domain".match(/(?<=@)\w+/) // ["domain"]
// Negative lookbehind (?<!)
/(?<!\d)\d{3}(?!\d)/ // 3 digits not part of larger number
Named Groups
const pattern = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/;
const match = "2024-01-15".match(pattern);
console.log(match.groups.year); // "2024"
console.log(match.groups.month); // "01"
console.log(match.groups.day); // "15"
Non-Greedy Matching
// Greedy (default): matches as much as possible
/<.>/g.exec("<a><b>") // ["<a><b>"]
// Non-greedy: matches as little as possible
/<.
?>/g.exec("<a><b>") // ["<a>"]Practical Examples
// Password validation (complex)
/^(?=.[a-z])(?=.[A-Z])(?=.\d)(?=.[@$!%?&])[A-Za-z\d@$!%?&]{8,}$/
// Extract domain from URL
/(?<=:\/\/)[^/]+/
// Match balanced quotes
/"([^"\\]|\\.)*"/
// Validate credit card (basic)
/^\d{4}[- ]?\d{4}[- ]?\d{4}[- ]?\d{4}$/