Negative lookbehind on a capture group

You can exclude digits from the \S class, then surround the expression
with whitespace boundrys, then viola ..

(?<!\S)(?<PreRef>[^\s\d]+):(?<Ref>\d{5})(?!\S)

https://regex101.com/r/JrU7Kd/1

Explained

 (?<! \S )                     # Whitespace boundary
 (?<PreRef> [^\s\d]+ )         # (1), Not whitespace nor digit
 :                             # Colon
 (?<Ref> \d{5} )               # (2), Five digits
 (?! \S )                      # Whitespace boundary

Do you need a negative lookbehind? It's easier to just exclude digits from the PreRef capture. [^\W\d] will match word characters but not digits. Then you just need to add a \b or other similar word boundary assertion to make sure what does match is a full word.

\b(?<PreRef>[^\W\d]+):(?<Ref>\d{5})