Regex for the string

Not clear what you trying to do, but you also can try this regular expression

\b((X/)?X\|[0-9]\.[0-9]{2}\|S\|[0-9]{1})\b

Example: cp_reg_string between X|3.20|S|0 | C# Online Compiler | .NET Fiddle[^]

This one should do:

\sX\|3\.20\|S\|0

Just a few interesting links to help building and debugging RegEx.
Here is a link to RegEx documentation:
perlre - perldoc.perl.org[^]
Here is links to tools to help build RegEx and debug them:
.NET Regex Tester - Regex Storm[^]
Expresso Regular Expression Tool[^]
RegExr: Learn, Build, & Test RegEx[^]
Online regex tester and debugger: PHP, PCRE, Python, Golang and JavaScript[^]
This one show you the RegEx as a nice graph which is really helpful to understand what is doing a RegEx:
Debuggex: Online visual regex tester. JavaScript, Python, and PCRE.[^]
This site also show the Regex in a nice graph but can't test what match the RegEx:
Regexper[^]