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1.18. Escaping Characters

You need to output a string with
certain characters (quotes, commas, etc.) escaped. For instance,
you're producing a format string for sprintf and
want to convert literal % signs into
%%.

1.18.2. Solution

1.18.3. Discussion

The following code lets you do escaping when preparing strings to
submit to the shell. (In practice, you would need to escape more than
just ' and " to make any
arbitrary string safe for the shell. Getting the list of characters
right is so hard, and the risks if you get it wrong are so great,
that you're better off using the list form of
system and exec to run
programs, shown in Recipe 16.2. They avoid
the shell altogether.)

$string = q(Mom said, "Don't do that.");
$string =~ s/(['"])/\\$1/g;

We had to use two backslashes in the replacement because the
replacement section of a substitution is read as a double-quoted
string, and to get one backslash, you need to write two. Here's a
similar example for VMS DCL, where you need to double every quote to
get one through:

$string = q(Mom said, "Don't do that.");
$string =~ s/(['"])/$1$1/g;

Microsoft
command interpreters are harder to work with. In Windows,
COMMAND.COM recognizes double quotes but not
single ones, disregards backquotes for running commands, and requires
a backslash to make a double quote into a literal. Any of the many
free or commercial Unix-like shell environments available for Windows
will work just fine, though.

Because we're using character classes in the regular expressions, we
can use - to define a range and
^ at the start to negate. This escapes all
characters that aren't in the range A through Z.

$string =~ s/([^A-Z])/\\$1/g;

In practice, you wouldn't want to do that, since it would pick up a
lowercase "a" and turn it into
"\a", for example, which is ASCII BEL character.
(Usually when you mean non-alphabetic characters,
\PL works better.)

If you want to escape all non-word
characters, use the \Q and \E
string metacharacters or the quotemeta function.
For example, these are equivalent:

$string = "this \Qis a test!\E";
$string = "this is\\ a\\ test\\!";
$string = "this " . quotemeta("is a test!");

1.18.4. See Also

The s/// operator in
perlre(1) and perlop(1) and
Chapter 5 of Programming Perl; the
quotemeta function in
perlfunc(1) and Chapter 29 of
Programming Perl; the discussion of HTML
escaping in Recipe 19.1; Recipe 19.5 for how to avoid having to escape strings to
give the shell

1.17. Reformatting Paragraphs		1.19. Trimming Blanks from the Ends of a String

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