You might read it as this: 

s/
  (?<!        # 2. NOT preceded by
     \pL      #    a letter
  )

  (           # 1. Capture
     \pL      #    a letter
  )

  (           # 3. Capture
     \(       #    open paren;
     [\d+-]+  #    one or more digits, plus, minus;
     \)       #    close paren
  )?          # (Zero or one time)

  (?!         # 4. NOT followed by
     \pL      #    a letter
  )

/(${1}SS*exp($1$2))/gx;

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In reply to Re^3: Help with regex - find captured pattern twice by hbm
in thread Help with regex - find captured pattern twice by pachydermic

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