There are few criteria in this file that you can test for easily and fast:

• Every line starts with three upper case ASCII letters (m/^[A-Z]{3}/)
• Lines beginning with R</a> always follow the pattern <c>m/^R[A-Z]{2}(?:[A-Z]|\s*\d+)$/
• limited character set: The sample file contains only word characters (\w), whitespaces and [()-]

I don't know if these characteristics are binding, but if they are they could be used to identify such a file after reading some 20 or 50 lines.