Experimental lexer generator: Fix byte order mark detection.

src/lexer/lexer_py.re

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 whitespace_char = [:whitespace:];
-whitespace = whitespace_char+;
+byte_order_mark_char = [:byte_order_mark:];
+whitespace = whitespace_char+|byte_order_mark_char+;
 identifier_start = [$_:letter:];
 identifier_char = [:identifier_start::identifier_part_not_letter:];
 line_terminator = [:line_terminator:];
 digit = [0-9];
 hex_digit = [0-9a-fA-F];
 single_escape_char = ['"\\bfnrtv];
 maybe_exponent = /([eE][\-+]?[:digit:]+)?/;
 number =
   /0[xX][:hex_digit:]+/ | (
   /\.[:digit:]+/ maybe_exponent |
@@ skipping 189 matching lines @@
 eos <|skip_and_terminate|>
 catch_all <||continue>
 
 <<MultiLineComment>>
 /\*+\//             <|skip|>
 # TODO find a way to generate the below rule
 /\*+[^\/*]/       <||continue>
 line_terminator  <push_line_terminator||continue>
 eos <|terminate_illegal|>
 catch_all        <||continue>