Perl - Regular Expressions - $value =~ s/~!/ ~!/g;?

Question

This line is from the classic formmail.cgi script:
$value =~ s/~!/ ~!/g;

It's to stop people from using subshells.  But, does anyone know what this line is actually doing and how it's doing it?

Thanks.

martinthurn · Accepted Answer

In the Unix email reader program mailx, when you type ~! at the front of a line, it opens a subshell.  Using this regex, formmail.cgi is inserting a space in front of every occurrence of tilde-bang, so that it never looks like it's at the front of a line (and therefore mailx will never open a subshell, which would be one way for someone to hack into your computer).

? · Answer

you may not do it right now. you ought to examine the record right into a string. use strict; use warnings; use record::Slurp; my $s = read_file(q{filename.txt}); $s =~ s/expression/alternative/g; write_file(q{filename.txt}, $s); __END__ or you would be able to desire to attempt the command-line arguments: perl -ibak -pe"s/expr/repl/g" filename.txt yet you have to be careful with quoting the characters in the regex, and so on.

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Perl - Regular Expressions - $value =~ s/~!/ ~!/g;?

2 Answers