Regular Expression Interpolation

An interpolated regular expression is a regular expression that is compiled and evaluated at runtime. Today all regular expressions are compiled at configuration time in order to improve performance. There are cases where a regular expression is determined dynamically from data within a SIP message. In these circumstances the regular expression is unknown until the time of execution.

In order to have a regular expression be interpolated at runtime, it must be contained within a set of {}. An interpolated expression can have any number of regular expressions and strings appended together. Any characters to the left or right of the curly braces will be appended to the value within the curly braces. The curly braces are effectively two operators treated as one (interpolate the value contained within and then concatenate the values to the left and right of the curly braces). If the comparison-type is set to pattern-rule and the match-value contains a value that matches the grammar below, then it will be treated as an interpolated expression.

([^\\]|^)\{\$[^0-9]+[^}]*\}

The example below demonstrates using a user defined variable within a regular expression of another rule at runtime.

element-rule

        name                           someRule
        type                           header-value
        action                         replace
        comparison-type                pattern-rule
        match-value                    ^sip:{$rule1.$0}@(.+)$
        new-value                      sip:[e-mail]

If the value of $rule1.$0 evaluates to alice then it will successfully match against the string sip:[e-mail]. An interpolated expression can be as simple as “{$rule1.$0}” or as complex as ^sip:{rule1.$0}@{$rule2[1].$2}$. It is not possible to interpolate a normal regular expression since the grammar will not allow the user to enter such an expression. Only variables can be contained with the curly braces.

The resultant of interpolated expressions can be stored in user defined variables. Given the same example from above, if the rule someRule was referenced by another rule, the value of sip:[e-mail] would be stored within that rule.

Interpolation only makes a single pass at interpolation, but does so every time the Rule executes. In other words, if the Rule is applied to the Route header, it will interpolate again for each Route header instance. What this means is that the value within the curly braces will only be evaluated once. For instance, if the value {$someRule.$1} evaluates to {$foobar.$2} the Oracle® Enterprise Session Border Controller (E-SBC) will treat $foobar.$2 as a literal string which it will compile as a regular expression. The E-SBC will not recursively attempt to evaluate $foobar.$2, even if it was a valid user defined variable.

Interpolated regular expressions will evaluate to TRUE if an only if both the regular expression itself can be compiled and it successfully matches against the compared string.