Optimize regex formula for checking filenames

Question

I'm working on a regex formula that will check if the supplied file for processing has either of the following formats:

1. SN1234_filename.pdf
2. SN1234_filename.pdf.zip

This is what I have right now:

MYFILE="SN39586_invoice.pdf"  
ISZIP=0  
if [ $ISZIP -eq 0 ]; then  
    FORMAT='^SN[0-9]+\_[a-zA-Z0-9]+\\.pdf$'  
    else  
        FORMAT='^SN[0-9]+\_[a-zA-Z0-9]+\\.pdf\\.zip$'  
    fi

if [[ $MYFILE =~ $FORMAT ]]; then  
  # Do some processing  
  else  
  echo "invalid file format"  
fi

This is working currently, but I want to get rid of the if-else block that sets the FORMAT variable. How can I optimize the FORMAT variable?

Answer 1

Use the ? operator, which matches zero or one occurence of the previous character or group:

^SN[0-9]+_[a-zA-Z0-9]+\.pdf(\.zip)?$