JS: RegExp Syntax

This page covers regex pattern syntax.

(For basic examples of how to use regex, see JS: RegExp Tutorial)

Regex pattern has 2 parts:

• pattern → the pattern to match.
• flags → flags is used to control how a pattern is matched or how the regex function behaves. For example, ignore letter case.

RegExp Flags

Regex flags are specified in 2 ways:

• Literal expression: /pattern/flags
• Constructor: RegExp("pattern", "flags")

[see JS: RegExp Constructor]

Regex flag changes the meaning of the pattern, or the behavior of the regex function.

[see JS: Regex Find/Replace]

Example of using regex flag.

// check if the string contains pattern
console.log( "WATER".search( /t/ ) ); // -1

// with flag to ignore case
console.log( "WATER".search( /t/i ) ); // 2

// return value is the start position of match, or -1 if not found

Special Escapes for Literal Characters

[see ASCII Characters ␀ ␣ ¶]

// example of RegExp matching unicode by hex codepoint

console.log( "alpha α".search(/\u03b1/)); // 6

console.log( "alpha α".search(/α/)); // 6
// literal Unicode char is ok too

Character Sets, Character Classes

// check if string contain digit

console.log (
 "xyz 123".search( /\d/ )
); // 4

Boundaries

// example of regex with boundary check

console.log ( "something".search( /thing/ ) ); // 4

// check for “thing” if it's a word by itself
console.log ( "something".search( /\bthing\b/ ) ); // -1

Repetition

Note: these will match as far as possible. For non-greedy version, add a ? after them.

// example of regex repetition pattern

const str = "is 278";

// check if contains 1 or more digits
console.log ( str.search( /\d+/ ) ); // 3

// check if contains 4 or more digits
console.log ( str.search( /\d{4}/ ) ); // -1

Alternate and Conditions

// check if the string contains “water” or “fire”

const str = "some fire";

console.log( str.search( /water|fire/ ) ); // -5

Capture Group, Back Reference

For example of capture, see JS: String.prototype.match