is, if they are invariant. On ASCII-ish machines, only ASCII characters
fit this definition, hence the function's name.
+If C<len> is 0, it will be calculated using C<strlen(s)>.
+
See also is_utf8_string(), is_utf8_string_loclen(), and is_utf8_string_loc().
=cut
=for apidoc is_utf8_string
Returns true if first C<len> bytes of the given string form a valid
-UTF-8 string, false otherwise. Note that 'a valid UTF-8 string' does
-not mean 'a string that contains code points above 0x7F encoded in UTF-8'
-because a valid ASCII string is a valid UTF-8 string.
+UTF-8 string, false otherwise. If C<len> is 0, it will be calculated
+using C<strlen(s)>. Note that 'a valid UTF-8 string' does not mean 'a
+string that contains code points above 0x7F encoded in UTF-8' because a
+valid ASCII string is a valid UTF-8 string.
See also is_ascii_string(), is_utf8_string_loclen(), and is_utf8_string_loc().