具有全局多行无限匹配的RegExp

Question

Ok so i was trying to match all lines and obtain the string index of the matches but i ran into a weird issue

var string = 'a\n',
    regexp = /^.*$/gim;

if you run

regexp.exec(string);

It will eventually match "" infinitely, why? The reason i ask is because its common for people to perform these type of matches in a while loop and this will produce an infinite loop unless you are ensuring that the previous match index is not the current index.

Edit so the solution to my issue required me doing

while (match = regexp.exec(string)) {
    if (!match[0].length) {
        regexp.lastIndex++;
    }
}

Answer 1

Because .* matches a zero-length string. At the end of the string, it can always match this again.

Answer 2

It's matching the end of the string. When the m flag is used, ^ equals the start of a line... and you got a new line at the end, and the lastIndex (the value that's used to start the search from) won't move from that, because it matched an empty string.

To better understand this, try using this:

var string = "a\n\n\b\n";

It will never match "b" either.

Answer 3

Your string variable has 2 NEWLINE characters, resulting in a 3 line string

a
b
<<an empty line>>

Your RegEx states that you're searching for strings that matches entire lines, .* means any character 0 or more times

Since an empty line matches that .* Regex, you'll end up with the empty string as result.