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Find non-negative integers

There’s a string of integer numbers.

Create a regexp that looks for only non-negative ones (zero is allowed).

An example of use:

let str = "0 12 -5 123 -18";

alert( str.match(regexp) ); // 0, 12, 123

The regexp for an integer number is \d+.

We can exclude negatives by prepending it with the negative lookbehind: (?<!-)\d+.

Although, if we try it now, we may notice one more “extra” result:

let regexp = /(?<!-)\d+/g;

let str = "0 12 -5 123 -18";

console.log( str.match(regexp) ); // 0, 12, 123, 8

As you can see, it matches 8, from -18. To exclude it, we need to ensure that the regexp starts matching a number not from the middle of another (non-matching) number.

We can do it by specifying another negative lookbehind: (?<!-)(?<!\d)\d+. Now (?<!\d) ensures that a match does not start after another digit, just what we need.

We can also join them into a single lookbehind here:

let regexp = /(?<![-\d])\d+/g;

let str = "0 12 -5 123 -18";

alert( str.match(regexp) ); // 0, 12, 123