compare-strings is a built-in function in `C source code'.

(compare-strings STR1 START1 END1 STR2 START2 END2 &optional IGNORE-CASE)

Compare the contents of two strings, converting to multibyte if needed.
The arguments START1, END1, START2, and END2, if non-nil, are
positions specifying which parts of STR1 or STR2 to compare. In
string STR1, compare the part between START1 (inclusive) and END1
(exclusive). If START1 is nil, it defaults to 0, the beginning of
the string; if END1 is nil, it defaults to the length of the string.
Likewise, in string STR2, compare the part between START2 and END2.
Like in `substring', negative values are counted from the end.

The strings are compared by the numeric values of their characters.
For instance, STR1 is "less than" STR2 if its first differing
character has a smaller numeric value. If IGNORE-CASE is non-nil,
characters are converted to lower-case before comparing them. Unibyte
strings are converted to multibyte for comparison.

The value is t if the strings (or specified portions) match.
If string STR1 is less, the value is a negative number N;
- 1 - N is the number of characters that match at the beginning.
If string STR1 is greater, the value is a positive number N;
N - 1 is the number of characters that match at the beginning.