How to check in elisp if a string is a substring of another string?

Question

How to check if a string s1 is a substring of another string s2?

For example (test-substring "f t" "df tj") --> t, (test-substring "ft" "df tj") --> nil.

Answer 1

The standard Emacs Lisp approach is regular expression matching:

(string-match-p (regexp-quote needle) haystack)

Answer 2

cl-search can do that (and also returns the index of the substring, if found):

ELISP> (cl-search "f t" "df tj")
1 (#o1, #x1, ?\C-a)
ELISP> (cl-search "ft" "df tj")
nil