"ab".rindex(/ab/, 0) => nil

“ab”.rindex(/ab/, 0) e$B$,e(B nil e$B$K$J$k$N$O0U?^$5$l$?5sF0$G$7$g$&e(B
e$B$+!#e(B

% ./ruby -ve ‘p “ab”.rindex(/ab/, 0)’
ruby 1.9.0 (2007-07-19 patchlevel 0) [i686-linux]
nil

1.8 [email protected]$He(B 0 e$B$K$J$j$^$9!#e(B

% ruby-1.8 -ve ‘p “ab”.rindex(/ab/, 0)’
ruby 1.8.6 (2007-07-17 patchlevel 5000) [i686-linux]
0

e$B$^$?!“e(B"ab”.rindex(“ab”, 0) e$B$be(B 0 e$B$K$J$j$^$9!#e(B

% ./ruby -e ‘p “ab”.rindex(“ab”, 0)’
0