The answer lies in the fact that the shell does concatenation by absense of whitespace. So, if we want to put O'Reilly inside quotes, we break the string into three parts: O, ' and Reilly. The first and last part will be surrounded by single quotes, the single quote will be surrounded by double quotes, and then we stick the three parts together, resulting in: 'O'"'"'Reilly'. This sounds complicated, but we can do this with a single substitution: s/'/'"'"'/g.

#!/usr/bin/perl 
    
use strict;
use warnings;
    
while (<DATA>) {
    chomp;
    my $orig = $_;
    
    s/'/'"'"'/g;
    my $result = `echo '$_' | cat`;
    chomp $result;

    print STDERR "Mismatch [$orig] vs [$result]\n" unless $result eq $
+orig;
}

__DATA__   
foo bar  
'foo bar' 
foo'"'bar 
''''''''''''
'''''''''''
"foo'bar"  
"foo''bar"   
"foo'bar""
'
"'"
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Abigail

And this is why you should be thankful that system() and qx() use only /bin/sh and never "the user's preferred shell", because /bin/csh cannot quote all possible strings. I can't recall the combo you can't get now, but I think it's a backslash followed by a newline that cannot be quoted, because one backslash and a newline just creates a newline, but two backslashes and a newline creates two backslashes and a newline, or something like that.

-- Randal L. Schwartz, Perl hacker
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