Perl Cookbook

Escaping Characters

Problem

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 %%.

Solution

Use a substitution to backslash or double each character to be escaped.

# backslash
$var =~ s/([CHARLIST])/\\$1/g;

# double
$var =~ s/([CHARLIST])/$1$1/g;

Discussion

$var is the variable to be altered. The CHARLIST is a list of characters to escape and can contain backslash escapes like \t and \n. If you just have one character to escape, omit the brackets:

$string =~ s/%/%%/g;

The following 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 Section 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 ...