Substitution string element. Optional in
Defines a replacement string for the matched string or sub-string in the path or query.
If the pattern expression includes sub-expressions (delimited with parentheses), the first matched sub-string is replaced with the substitution string. If the pattern expression does not include sub-expressions, the entire matched string is substituted.
<expression> is empty or absent, the substitution string is appended to the path or query.
<substitution> is empty, the matched string or sub-string is removed. If
<substitution> is not specified, the path or query string is not modified.
The substitution string must not contain literal < and & characters. These reserved characters can be encoded with
<, respectively, or the entire string can be enclosed in an XML
<substitution><![CDATA[&text=<Hello, world!>]]></ substitution>