FAQ 4.6 Why doesn't & work the way I want it to?

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This is an excerpt from the latest version perlfaq4.pod, which
comes with the standard Perl distribution. These postings aim to
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to review and update the answers. The latest version of the complete
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4.6: Why doesn't & work the way I want it to?

The behavior of binary arithmetic operators depends on whether they're
used on numbers or strings. The operators treat a string as a series of
bits and work with that (the string "3" is the bit pattern 00110011).
The operators work with the binary form of a number (the number 3 is
treated as the bit pattern 00000011).

So, saying "11 & 3" performs the "and" operation on numbers (yielding
3). Saying "11" & "3" performs the "and" operation on strings (yielding
"1").

Most problems with "&" and "|" arise because the programmer thinks they
have a number but really it's a string. The rest arise because the
programmer says:

if ("\020\020" & "\101\101") {
# ...
}

but a string consisting of two null bytes (the result of ""\020\020" &
"\101\101"") is not a false value in Perl. You need:

if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
# ...
}



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