Strings can be specified using one of two sets of delimiters.

If the string is enclosed in double-quotes ("), variables within the string will be expanded (subject to some parsing limitations). As in C and Perl, the backslash ("\") character can be used in specifying special characters:

Table 6-1. Escaped characters

\thorizontal tab
\$dollar sign

You can escape any other character, but a warning will be issued at the highest warning level.

The second way to delimit a string uses the single-quote ("'") character, which does not do any variable expansion or backslash processing (except for "\\" and "\'" so you can insert backslashes and single-quotes in a singly-quoted string).

String conversion

When a string is evaluated as a numeric value, the resulting value and type are determined as follows.

The string will evaluate as a double if it contains any of the characters '.', 'e', or 'E'. Otherwise, it will evaluate as an integer.

The value is given by the initial portion of the string. If the string starts with valid numeric data, this will be the value used. Otherwise, the value will be 0 (zero). Valid numeric data is an optional sign, followed by one or more digits (optionally containing a decimal point), followed by an optional exponent. The exponent is an 'e' or 'E' followed by one or more digits.

When the first expression is a string, the type of the variable will depend on the second expression.

$foo = 1 + "10.5";              // $foo is double (11.5)
$foo = 1 + "-1.3e3";            // $foo is double (-1299)
$foo = 1 + "bob-1.3e3";         // $foo is integer (1)
$foo = 1 + "bob3";              // $foo is integer (1)
$foo = 1 + "10 Small Pigs";     // $foo is integer (11)
$foo = 1 + "10 Little Piggies"; // $foo is integer (11)
$foo = "10.0 pigs " + 1;        // $foo is integer (11)
$foo = "10.0 pigs " + 1.0;      // $foo is double (11)     

For more information on this conversion, see the Unix manual page for strtod(3).