Perl string quotation operators (q, qq, qr and qx)

In scripting programming languages such as Perl, PHP, and Python use string quotation extensively. To include a single or double quote inside the string quotation, you'll have to escape the character with a backslash. Perl string quotation operators (q, qq and qx) let you avoid putting too many backslashes into quoted strings.

q/STRING/, or q(STRING) - Single Quotes, does not allow interpolation.
qq/STRING/, or qq(STRING) - Double Quotes, allow interpolation.
qr/EXPR/, or qr(EXPR) - Regexp-like quote
qx/STRING/, or qx(STRING) - Backquote, executes external command inside backquotes.

The difference between a single quote and a double quote is an interpolation. A double quote allows the string to be replaced with the value of that variable inside the quote, while a single quote doesn't allow it.

The q operator (single quote) example:

#!/usr/bin/perl -w
$name = "Cindy";
$str = q/This is $name's dog./;
print $str;

The above script prints the following output.

This is $name's dog.

The qq operator (double quote) example:

#!/usr/bin/perl -w
$name = "Cindy";
$str = qq(This is $name's dog.);
print $str;

The above script prints the following output.

This is Cindy's dog.

The qr operator (regex quote) example:

#!/usr/bin/perl -w
$regex = qr/[io]n/i;
print $regex, "\n";
$string = "I am on chair in the room.";
$string =~ s/$regex/XX/mg;
print $string, "\n";

The above script prints the following output.

I am XX chair XX in the room.

Regular Expression Options:

m   Treat string as multiple lines.
s   Treat string as a single line. (Make . match a newline)
i   Do case-insensitive pattern matching.
x   Use extended regular expressions.
p   When matching preserve a copy of the matched string so
     that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
o   Compile the pattern only once.

The qx operator (backquote) example:

#!/usr/bin/perl -w
$str = qx(date);
print $str;

The above script prints the following output.

Wed May 18 19:30:36 CDT 2011

If the delimiter is an opening bracket or parenthesis, the final delimiter will be the corresponding closing bracket or parenthesis.