# File lib/randexp/parser.rb, line 3
    def self.parse(source)
      case
      when source =~ /^(.*)(\*|\*\?|\+|\+\?|\?)$/ && balanced?($1, $2)
        parse_quantified($1, $2.to_sym)                                 # ends with *, +, or ?: /(..)?/
      when source =~ /^(.*)\{(\d+)\,(\d+)\}$/ && balanced?($1, $2)
        parse_quantified($1, ($2.to_i)..($3.to_i))                      #ends with a range: /(..){..,..}/
      when source =~ /^(.*)\{(\d+)\}$/ && balanced?($1, $2)
        parse_quantified($1, $2.to_i)                                   #ends with a range: /..(..){..}/
      when source =~ /^\((.*)\)\((.*)\)$/ && balanced?($1, $2)
        union(parse($1), parse($2))                                     #balanced union: /(..)(..)/
      when source =~ /^(\(.*\))\|(\(.*\))$/ && balanced?($1, $2)
        intersection(parse($1), parse($2))                              #balanced intersection: /(..)|(..)/
      when source =~ /^(.*)\|(.*)$/ && balanced?($1, $2)
        intersection(parse($1), parse($2))                              #implied intersection: /..|../
      when source =~ /^(.*)\|\((\(.*\))\)$/ && balanced?($1, $2)
        intersection(parse($1), parse($2))                              #unbalanced intersection: /(..)|((...))/
      when source =~ /^(.+)(\(.*\))$/ && balanced?($1, $2)
        union(parse($1), parse($2))                                     #unbalanced union: /...(...)/
      when source =~ /^\((.*)\)$/ && balanced?($1)
        union(parse($1))                                                #explicit group: /(..)/
      when source =~ /^([^()]*)(\(.*\))$/ && balanced?($1, $2)
        union(parse($1), parse($2))                                     #implied group: /..(..)/
      when source =~ /^(.*)\[\:(.*)\:\]$/
        union(parse($1), random($2))                                    #custom random: /[:word:]/
      when source =~ /^(.*)\\([wsdc])$/
        union(parse($1), random($2))                                    #reserved random: /..\w/
      when source =~ /^(.*)\\(.)$/ || source =~ /(.*)(.|\s)$/
        union(parse($1), literal($2))                                   #end with literal or space: /... /
      else
        nil
      end
    end