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


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

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

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.


Example in Javascript to explain:

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

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

g is the global search flag.

The global search flag makes the RegExp search for a pattern throughout the string, creating an array of all occurrences it can find matching the given pattern.

So the difference between /.+/g and /.+/ is that the g version will find every occurrence instead of just the first.


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.


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);


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


G in regular expressions is a defines a global search, meaning that it would search for all the instances on all the lines.


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,'');

  1. g -> returns all matches
  2. without g -> returns first match

example:

  1. '1 2 1 5 6 7'.match(/\d+/) returns ["1", index: 0, input: "1 2 1 5 6 7", groups: undefined]. As you see we can only take first match "1".
  2. '1 2 1 5 6 7'.match(/\d+/g) returns an array of all matches ["1", "2", "1", "5", "6", "7"].