Regular Expressions in JavaScript: A Practical Guide

// Character classes
\\d    // digit [0-9]
\\w    // word char [a-zA-Z0-9_]
\\s    // whitespace
.     // any char except newline
[abc] // a, b, or c
[^abc] // NOT a, b, or c

// Quantifiers
*     // 0 or more
+     // 1 or more
?     // 0 or 1
{3}   // exactly 3
{2,5} // 2 to 5

// Anchors
^     // start of string
$     // end of string
\\b    // word boundary

// Email validation (simplified)
const email = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}$/;

// URL extraction
const urls = /https?:\\/\\/[^\\s]+/g;

// Phone number (flexible)
const phone = /\\+?\\d{1,3}[-.\\s]?\\(?\\d{1,4}\\)?[-.\\s]?\\d{1,4}[-.\\s]?\\d{1,9}/;

// Password strength (min 8 chars, uppercase, lowercase, digit, special)
const strongPassword = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\\d)(?=.*[@$!%*?&])[A-Za-z\\d@$!%*?&]{8,}$/;

// HTML tag stripping
const stripped = html.replace(/<[^>]*>/g, "");

const dateRegex = /(?<year>\\d{4})-(?<month>\\d{2})-(?<day>\\d{2})/;
const match = "2024-01-15".match(dateRegex);
console.log(match.groups.year);  // "2024"
console.log(match.groups.month); // "01"

// Positive lookahead: match "foo" followed by "bar"
/foo(?=bar)/ // matches "foo" in "foobar", not in "foobaz"

// Negative lookahead: match "foo" NOT followed by "bar"
/foo(?!bar)/ // matches "foo" in "foobaz", not in "foobar"

// Lookbehind
/(?<=\\$)\\d+/ // match digits preceded by $: "$100" → "100"