Regex Expression for finding particular number

test it on regex101 and engine set to .Net or .NET Regex Tester - Regex Storm

Some regex engines are not supporting conditional parts within the (?<= …)

Another quick test would the debugging and working within the immediate panel, as it is done directly in UiPath

Understanding the 6 Debugging Panels of UiPath in the easiest way possible! - News / Tutorials - UiPath Community Forum