The CountCharacterOccurrences( s, ch ) calling sequence counts the number of occurrences of the character ch in the string s. A non-negative integer is returned.

All of the StringTools package commands treat strings as (null-terminated) sequences of $8$-bit (ASCII) characters.  Thus, there is no support for multibyte character encodings, such as unicode encodings.

$\mathrm{with}\left(\mathrm{StringTools}\right)\:$

$\mathrm{CountCharacterOccurrences}\left(''abcde''\,''a''\right)$

$1$

$\mathrm{CountCharacterOccurrences}\left(''abada''\,''a''\right)$

$3$

$\mathrm{CountCharacterOccurrences}\left(''''\,''a''\right)$

$0$

$\mathrm{CountCharacterOccurrences}\left(''xyz''\,''a''\right)$

$0$