find function for substring matching

[aman-godara]

Description

Signature: find(string, pattern, start(Optional) = 1, end(Optional) = len(string), occurrence(Optional) = 1, consider_overlapping(Optional)= .true.)

Output: an integer (index)

Returns the index at which occurrence'th occurrence of substring pattern is present in the input string string
between the index start and end, if not found returns 0.
consider_overlapping: comment #1 & comment #2

Prior Art

Fortran has an intrinsic function index (description) which does the same thing but has limited features.

PS: Some of the features proposed above may not be best to add to the stdlib at this moment, hence there is also a need to discuss the importance of these features. They are added in the issue to bring them into the discussion.

[aman-godara]

A higher level implementation of find function (based upon KMP algorithm) can be found here.

occurrence argument helps to find the nth occurrence the user is looking for. It also provides an option to consider_overlapping substrings or not while finding the nth occurrence.