The best replacement for lex is clark.

The best alternatives to lex are rated as following:
★★★★★  clark
★★★  flex
★★  yacc
★  jenna
★  jewel
★  lex
★  sicarus
  aklex
  seer
  crave
  jus
  5th
  tory

Your alternative for lex is not listed? Suggest!
Speaking about lex, one probably shouldn't forget about yacc.

Other search terms related to lex and their ratings are:
★★★★★ yacc
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Google Trends

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lex
clark
flex