Variable basics

Like in Perl, variable names begin with a dollar sign, followed by a letter or an underscore, optionally followed by more letters, digits, or underscores.

Also like in Perl, variables can contain strings.

Examples:

$five = 5;
$answer = ($five << 3)+2;
$rate = 135.5 Mbps;
$host = "www.foo.edu";

If an expression stored in a variable is not constant, it is inserted ``as is'' wherever the variable is referenced. Note that any references to fields or other variables are resolved at the time of assignment.

Example:

$cond = ip_src == 1.2.3.4 && tcp_sport == 80;
...
    class (1) if $cond;
...
field tcp_sport = ...;  // does not change $cond
...
    class (2) if $cond;