First, there is no "standard" regular expression specification. There are many different regular expression implementations with slightly different escaping rules and special features. The problem is that Curl does not support the more recently introduced (?<name>...) syntax for named subgroups. This was originally an extension introduced in the Python language's regular expression implementation, where it used the syntax (?P<name>...), and that is what we support in Curl (it is also supported in Perl). There are a number of other extensions in Perl that have not yet made it to Curl, so you should always check the Curl documentation to see if the feature you want to use is supported.

Also note that both Curl and Perl differ from .NET in that subgroups are numbered regardless of whether they are named, while in .NET they are not. So that given the expression

(?<foo>x)(y)

the group containing 'y' would be numbered 2 under Curl or Perl but would be numbered 1 under .NET.