What is the meaning of the 'g' flag in regular expressions?

Question

What is the meaning of the g flag in regular expressions?

What is is the difference between /.+/g and /.+/?

Answer 1

Example in Javascript to explain:

> 'aaa'.match(/a/g)
[ 'a', 'a', 'a' ]

> 'aaa'.match(/a/)
[ 'a', index: 0, input: 'aaa' ]

Answer 2

Will give example based on string. If we want to remove all occurences from a string. Lets say if we want to remove all occurences of "o" with "" from "hello world"

"hello world".replace(/o/g,'');

Answer 3

There is no difference between /.+/g and /.+/ because they will both only ever match the whole string once. The g makes a difference if the regular expression could match more than once or contains groups, in which case .match() will return an array of the matches instead of an array of the groups.

Answer 4

Beside already mentioned meaning of g flag, it influences regexp.lastIndex property:

The lastIndex is a read/write integer property of regular expression instances that specifies the index at which to start the next match. (...) This property is set only if the regular expression instance used the "g" flag to indicate a global search.

Reference: Mozilla Developer Network

Answer 5

As @matiska pointed out, the g flag sets the lastIndex property as well.

A very important side effect of this is if you are reusing the same regex instance against a matching string, it will eventually fail because it only starts searching at the lastIndex.

// regular regex
const regex = /foo/;

// same regex with global flag
const regexG = /foo/g;

const str = " foo foo foo ";

const test = (r) => console.log(
    r,
    r.lastIndex,
    r.test(str),
    r.lastIndex
);

// Test the normal one 4 times (success)
test(regex);
test(regex);
test(regex);
test(regex);

// Test the global one 4 times
// (3 passes and a fail)
test(regexG);
test(regexG);
test(regexG);
test(regexG);

Answer 6

g is for global search. Meaning it'll match all occurrences. You'll usually also see i which means ignore case.

Reference: global - JavaScript | MDN

The "g" flag indicates that the regular expression should be tested against all possible matches in a string.

Without the g flag, it'll only test for the first.