Different function output based on order of functions

Question

I'm practicing with regular expressions. I found a great answer here on StackOverflow: How do I retrieve all matches for a regular expression in JavaScript?

I copied the code from the checked answer and pasted it into an editor and then wrote my own function using it. So here's my JS file:

const regex = /[a-zA-Z]/g;
const _string = "++a+b+c++";

//The following is the loop from the StackOverflow user

var m;
do {
  m = regex.exec(_string);
  if (m) {
    // Logging the results
    console.log(m, "hello");
  }
} while (m);

console.log("Separating top and bottom so it's easier to read");
// Now here is the function I wrote using that code

const match = str => {
  let _matches;
  do {
    _matches = regex.exec(_string);
    if (_matches) {
      // Logging the results
      console.log(_matches);
    }
  } while (_matches);
}

match(_string);

Here is my problem: when I run this code (it's a Repl.it) the results from the function that is first (so in this case, the loop from the stackoverflow user) do not include the first match return from the RegExp.prototype.exec() method. Here is my console output:

 node v10.15.2 linux/amd64

 [ 'b', index: 4, input: '++a+b+c++', groups: undefined ] 'hello'
 [ 'c', index: 6, input: '++a+b+c++', groups: undefined ] 'hello'
 Separating top and bottom so it's easier to read
 [ 'a', index: 2, input: '++a+b+c++', groups: undefined ]
 [ 'b', index: 4, input: '++a+b+c++', groups: undefined ]
 [ 'c', index: 6, input: '++a+b+c++', groups: undefined ]

And if I switch the order of the loop and the function, the function will not return the first match and loop will return all three.

Any help would be much appreciated!

Just to clarify: is it the question about the regexp or the functions output? I mean that the problem can be solved much easier if it is about regexp. — Ilia Liachin, Jun 14 '19 at 19:29
Also, I just tried it in the browser's console. And everything works as expected. Maybe the problem in the `repl.it`. Node also returns correct result. — Ilia Liachin, Jun 14 '19 at 19:30
I suspect the problem is that you called `regex.exec()` by hand outside the loop before you ran the loop. So inside the loop it started from the 2nd match. — Barmar, Jun 14 '19 at 19:34
I called regex.exec() inside the function and it worked, thank you — Jack Surtees, Jun 14 '19 at 19:42

score 1 · Answer 1 · answered Jun 14 '19 at 19:33

1

If you want to get letters-only from the sttring, you don't need any loop.
You can do

"++a+b+c+A+".match(/[a-z]/gi)

answered Jun 14 '19 at 19:33

Ilia Liachin

1,284
2
17
35

I'm trying to get the index of each match. Sorry i shoulda said that in my initial post – Jack Surtees Jun 14 '19 at 19:42

score 1 · Accepted Answer · answered Jun 14 '19 at 22:55

I'm guessing that we might be dealing with strings here, and we wish to get the numeric index where ++a+b+c++ is present, which this expression might likely work:

index:\s*([0-9]+)\s*,.+'(\+\+a\+b\+c\+\+)'

Demo 1

const regex = /index:\s*([0-9]+)\s*,.+'(\+\+a\+b\+c\+\+)'/gm;
const str = `node v10.15.2 linux/amd64

 [ 'b', index: 4, input: '++a+b+c++', groups: undefined ] 'hello'
 [ 'c', index: 6, input: '++a+b+c++', groups: undefined ] 'hello'
 Separating top and bottom so it's easier to read
 [ 'a', index: 2, input: '++a+b+c++', groups: undefined ]
 [ 'b', index: 4, input: '++a+b+c++', groups: undefined ]
 [ 'c', index: 6, input: '++a+b+c++', groups: undefined ]`;
let m;

while ((m = regex.exec(str)) !== null) {
    // This is necessary to avoid infinite loops with zero-width matches
    if (m.index === regex.lastIndex) {
        regex.lastIndex++;
    }
    
    // The result can be accessed through the `m`-variable.
    m.forEach((match, groupIndex) => {
        console.log(`Found match, group ${groupIndex}: ${match}`);
    });
}

Or if we just wish to only get the indices,

index:\s*([0-9]+)\s*,

const regex = /index:\s*([0-9]+)\s*,/gm;
const str = `node v10.15.2 linux/amd64

 [ 'b', index: 4, input: '++a+b+c++', groups: undefined ] 'hello'
 [ 'c', index: 6, input: '++a+b+c++', groups: undefined ] 'hello'
 Separating top and bottom so it's easier to read
 [ 'a', index: 2, input: '++a+b+c++', groups: undefined ]
 [ 'b', index: 4, input: '++a+b+c++', groups: undefined ]
 [ 'c', index: 6, input: '++a+b+c++', groups: undefined ]`;
let m;

while ((m = regex.exec(str)) !== null) {
    // This is necessary to avoid infinite loops with zero-width matches
    if (m.index === regex.lastIndex) {
        regex.lastIndex++;
    }
    
    // The result can be accessed through the `m`-variable.
    m.forEach((match, groupIndex) => {
        console.log(`Found match, group ${groupIndex}: ${match}`);
    });
}

Different function output based on order of functions

2 Answers2

Demo 1

Demo 2