The substring-before() function returns that part of an input string that precedes the first occurrence of a comparison string in the input string.
The comparison string should consist of at least one single character or a character string.
If the comparison string occurs more than once in the examined string - for example a separator in a list available as a single string - only its first occurrence is considered.

substring-before('String', 'Comparison String')

substring-before('SLSRPT_1324.csv,'.csv')

Returns SLSRPT_1324