That optimization came with 5.10.

From man perl5100delta: 

       Single char char-classes treated as literals
           Classes of a single character are now treated the same as i
+f the
           character had been used as a literal, meaning that code tha
+t uses
           char-classes as an escaping mechanism will see a speedup. (
+Yves
           Orton)

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In reply to Re^6: How we can separate a backref from a digit? by JavaFan
in thread How we can separate a backref from a digit? by Serge314

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