Hi,
Is some of your code missing? Regex.Replace takes three arguments, your code snippet only has 2, and the regular expression string argument #2
"(<[a|A][^>]*>
isn't terminated. (looks like it needs a closing parenthesis, and a closing double-quote.)
Regex101.com is a great site for testing Regular Expression matching.
If you want the code to run - it looks like C# to me, so create a new Console Application, paste the code in, and run it.
I had success by finishing your "Replace" line as follows:
str1 = System.Text.RegularExpressions.Regex.Replace(str1, "(<[a|A][^>]*>)", "test");
The effect was that all opening anchor tags <a href="......"> were replaced with the string "test". Closing anchor tags were not replaced, neither were the tag contents. Is that what you expected?
Good luck with it....