regex - JavaScript regular expression with global match help needed

Question

I'm new to JavaScript and have a question about regular expressions. I have the following code:

var patt3=new RegExp(/(July|August)s+d{1,2}(s|,)d{4}/g);
var str3 = "August                               12,1988";
var match3 = str3.match(patt3);
document.write(match3.toString() + "<br/>");

The output is: August 12,1988

Here is the same code but with the 'g' removed from the end of the RegExp:

var patt3=new RegExp(/(July|August)s+d{1,2}(s|,)d{4}/);
var str3 = "August                               12,1988";
var match3 = str3.match(patt3);
document.write(match3.toString() + "<br/>");

The output becomes: August 12,1988,August,,

From the definitions that I've found on the web, 'g' is supposed to match all occurrences of the patterns. But I'm still kind of confused as to what effect 'g' has on the code.

I would greatly appreciate any clarifications.

Thanks in advance.

Answer 1

The crucial difference is that the string.match method is defined to have a different behavior if the regex patter is global or not.

If the pattern is global, an array with all the matches. In your case you there is only one match but you can see the difference with an example like

let matches = "aaaa".match(/a(a)/g); 
console.log(matches) // returns ["aa", "aa"]