I was researching this too and found that Micro Focus recently published a KM article about this.

The article says that "Regex requires declaration at the top of the script:"

using System.Text.RegularExpressions;

And gives the following examples:

namespace Script
{
using System;
...
using System.Text.RegularExpressions;

[Serializable()]
public class TestUserCode : TestEntities
{
...

Then they give some sample code:

//Define variables
string sText;
bool bRxMchFnd; //true/false

//- Pattern NOT found
RxMatch = Regex.Match(sText, "xxx");
bRxMchFnd = RxMatch.Success; //False
this.Context.UserLogger.Info(" Match 'xxx': " + bRxMchFnd);

which gives the following output:

String: 'abcd'
Match 'bc': True
Match 'xxx': False