Extracts a module-specific search option from a search expression.
Search options are added using search_expression_insert(), and retrieved using search_expression_extract(). They take the form option:value, and are added to the ordinary keywords in the search expression.
$expression: The search expression to extract from.
$option: The name of the option to retrieve from the search expression.
The value previously stored in the search expression for option $option, if any. Trailing spaces in values will not be included.
function search_expression_extract($expression, $option) {
$expression = new SearchExpression($expression);
return $expression
->extract($option);
}