Regular expressions are used to get maximum flexibility when modifying HTTP requests. The concept of regular expressions is not specific to SAP, and you will find information about them in technical literature.
The POSIX standard is used here for regular expressions ("POSIX 1003.2, section 2.8 (Regular Expression Notation)").
Exception
The only exception to the standard is the use of "$" in the substitution expression.
With $n (with n = 0..9) the n-th expression, which in the search string is enclosed in parentheses as a group and was found in the search, can be referenced.
With string "/abc/index.html" and the regular expression "/a(.*)/(.*)" the results are: $1 = "bc" and $2 = "index.html"