in reply to Pattern matching regex problem
tr doesn't take character classes in square brackets ([]), so your code can also be written as:
if (0 == $s =~ tr/IOSM//c) {
Your code counts whether there is any of the letters I, O, S or M in the string. Which is obviously true.
Maybe you wanted to match the letters in order?
m/I+O+S+M+/
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