result = regexprep(YourCell, '^(STB|V).*?(\d+E\d+).*', '$2');
I used the rule from your "desired output", not the rule from your description: your desired output extracts the number from a string that starts with V but your description says only if it starts with STB. If the rule is to be STB only and the transformation of the V line was in error then
result = regexprep(YourCell, '^STB.*?(\d+E\d+).*', '$1')