Rewrite regex without positive look-behind

Question

I have the following regex that uses look-behind to match different time formats inside a string:

(?:(?<=[\s])|^)(2[0-3]|[01]?[0-9]):([0-5]?[0-9])(?::([0-5]?[0-9]))?( ?[AP]M)?(?:(?=[\s])|$)

The regex must match 12-hour or 24-hour time formats, with minutes and optional seconds. Examples:

10:00 AM
Time: 10:00 (only the `10:00` part)
13:23:34

The regex should entirely reject the following strings:

hello10:00 (the time is not separated by whitespace)
24:00:00 (24 does not make sense)
9912:1299 (there's a time substring, but the whole string is not really a time).

I'm trying to rewrite it to not use positive look-behind, which is not supported by every web browser. How could I do it?

Answer 1

const rgx = /\b(?:1?\d|2[0-3]):[0-5]\d(?::[0-5]\d)? ?(?:[ap]m)?\b/gi;

Segment	Description
\b	to the left is a non-word and to the right is a word
(?:1?\d	begin non-capturing group 0 to 19
|	OR
2[0-3])	20 to 23 end non-capturing group
:[0-5]\d	: 00 to 59
(?::[0-5]\d)? ?	same as above with a space or nothing
(?:[ap]m)?	a or p then m or nothing
\b	to the right is a non-word and to the left is a word

const rgx = /\b(?:1?\d|2[0-3]):[0-5]\d(?::[0-5]\d)? ?(?:[ap]m)?\b/gi

const str = `
10:00 AM
Time: 10:00
13:23:34
hello10:00
24:00:00
9912:1299`;

const res = [...str.matchAll(rgx)].flat();

console.log(res);

Answer 2

Your pattern can be modified to:

(?:^|[ \t])(2[0-3]|[01]?[0-9]):([0-5]?[0-9])(?::([0-5]?[0-9]))?( ?[AP]M)?(?:(?=[\s])|$)

Demo