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The precedence of the regular expression operators from highest to lowest is

  • A.concatenation, then closure, then alternation
  • B.alternation, then closure, then concatenation
  • C.closure, then concatenation, then alternation
  • D.alternation, then concatenation, then closure

Answer: C

Star binds tightest, then juxtaposition, then the vertical bar. This is why ab|c means the alternation of ab and c rather than a followed by the alternation of b and c. Parentheses are the only way to override the built in precedence of the operators.

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