Replace a long case statement involving regex with a hash

You are using a string as a key in your hash map instead of a regular expression object.
Here is the correct hash map declaration:

file_types = { /camt.053.001/ => 'camt053', /camt.052.001/ => 'camt052', /"Auftragskonto";"Buchungstag";"Valutadatum";/ => 'SpK' }

I would use a single regex and a hash code that does not use regex, just the output of a
single regex match to do the code lookup.

Demo : https://www.jdoodle.com/ia/1NBq

file_types = { 'camt.053.001' => 'camt053', 'camt.052.001' => 'camt052', '"Auftragskonto";"Buchungstag";"Valutadatum";' => 'SpK' } RxFileTypes = /(?:camt\.053\.001|camt\.052\.001\.08|"Auftragskonto";"Buchungstag";"Valutadatum";)/ first_bytes = '<?xml version="1.0" encoding="UTF-8"?><Document xmlns="urn:iso:std:iso:20022:tech:xsd:camt.053.001.02"' match = first_bytes.match( RxFileTypes ) if match puts "File type code is : " + file_types[ match[0] ] end

Output

File type code is : camt053

Combine all the file types into a single regex separated by alternations.
The match will be the Key into the File Type code hash.

(?: camt \. 053 \. 001 | camt \. 052 \. 001 \. 08 | "Auftragskonto";"Buchungstag";"Valutadatum"; )

For additional File types just add another alternation and an entry into the File Type code hash.