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JavaScript Regular Expressions

The RegExp object is used for matching text with a pattern.

There is two ways to create a RegExp object: a literal notation and a constructor.

let regexp = /ab+c/i; // literal notation
let regexp = new RegExp('ab+c', 'i') // constructor with string pattern as first argument
let regexp = new RegExp(/ab+c/, 'i') // constructor with regular expression literal as first argument (Starting with ECMAScript 6)

RegExp Syntax Table:

Regex Meaning
[abx-z] One character of: a, b, or the range x-z
[^abx-z] One character except: a, b, or the range x-z
^ Beginning of the string
$ End of the string
\d A digit (same as [0-9])
\D A non-digit (same as [^0-9])
\w A word character (same as [_a-zA-Z0-9])
\W A non-word character (same as [^_a-zA-Z0-9])
\s A whitespace character
\S A non-whitespace character
\b A word boundary
\B A non-word boundary
\n A newline
\t A tab
a | b a or b
a? Zero or one a's (greedy)
a?? Zero or one a's (lazy)
a* Zero or more a's (greedy)
a*? Zero or more a's (lazy)
a+ One or more a's (greedy)
a+? One or more a's (lazy)
a{4} Exactly 4 a's
a{4,8} Between (inclusive) 4 and 8 a's
a{9,} 9 or more a's
(?=...) A positive lookahead (if followed by)
(?!...) A negative lookahead (if not followed by)
(?:...) A non-capturing group
(...) A capturing group
\cY The control character with the hex code Y
\xYY The character with the hex code YY
\uYYYY The character with the hex code YYYY
. Any character
\Y The Y'th captured group

Simple RegExp examples

  • Check does string contain date(format: XX/XX/XXXX):
let regexp = /(1[0-2]|0?[1-9])\/(3[01]|[12][0-9]|0?[1-9])\/(?:[0-9]{2})?[0-9]{2}$/;
console.log(regexp.test('Albert Einstein was born in Ulm, on 14/03/2019'))
  • Check is string a time(format: XX:XX:XX):
let regexp = /^([01]?[0-9]|2[0-3]):([0-5]?[0-9]):([0-5]?[0-9])$/;
console.log(regexp.test('12:38:01'))
  • Pattern that matches e-mail addresses:
let regexp = /^[A-Z|a-z]+[0-9]*?[!#@\$%&\+-\/=\?\^_`\{\|\}~]+[A-Z|a-z]+[0-9]*?.\w{2,3}/
console.log(regexp.test("me-info@example.com"))
  • Check is string a hexcolor:
let regexp = /^#?([0-9a-fA-F]{3}|[0-9a-fA-F]{6})$/;
console.log(regexp.test("#00F"))
  • Add thousands separators:
function thousands_separators(num, separator){
    let value = num.toString().split('.');
    value[0] = value[0].replace(/\B(?=(\d{3})+(?!\d))/g, separator);
    return value.join(".");
}
console.log(thousands_separators(10000, ","))
  • Delete comment from code:
function stripCommeпts(code){
    return code.replace(/\/\/.*|\/\*[^]*?\*\//g, "");
};
console.log(stripCommeпts("1 /* а */+/* b */ 1"));
console.log(stripCommeпts("1 + 1 //a+b"));
  • Replace single qoute with double:
let regexp = /(^|\W)'|'(\W|$)/g;
let text = "'I'm the cook,' he said, 'it's my job.'";

console.log(text.replace(regexp, '$1"$2'));

More functions can be found in regexp.js file.

Usefull source for understanding RegExp: https://www.debuggex.com/