Support custom lex function

lib/propane/generator.rb

@@ -354,6 +354,14 @@ class Propane
       code
     end
 
+    # Get the lex function to use.
+    #
+    # @return [String]
+    #   Lex function to use.
+    def lex_fn
+      @grammar.lex_fn || "#{@grammar.prefix}lex"
+    end
+
     # Get the parser value type for the start rule.
     #
     # @return [Array<String>]

lib/propane/grammar.rb

@@ -6,6 +6,7 @@ class Propane
     IDENTIFIER_REGEX = /(?:[a-zA-Z]|_[a-zA-Z0-9])[a-zA-Z_0-9]*/
 
     attr_reader :context_user_fields
+    attr_reader :lex_fn
     attr_reader :tree
     attr_reader :tree_prefix
     attr_reader :tree_suffix
@@ -69,6 +70,7 @@ class Propane
       elsif parse_comment_line!
       elsif @modeline.nil? && parse_mode_label!
       elsif parse_context_user_fields_statement!
+      elsif parse_lex_fn!
       elsif parse_tree_statement!
       elsif parse_tree_prefix_statement!
       elsif parse_tree_suffix_statement!
@@ -117,6 +119,12 @@ class Propane
       end
     end
 
+    def parse_lex_fn!
+      if md = consume!(/lex_fn\b\s*(\w+)\s*;/)
+        @lex_fn = md[1]
+      end
+    end
+
     def parse_tree_statement!
       if consume!(/tree\s*;/)
         @tree = true