Without locale:
Compiling REx "\w"
Final program:
   1: ALNUM (2)
   2: END (0)
stclass ALNUM minlen 1 
Compiling REx "[\w]"
Final program:
   1: ANYOF[0-9A-Z_a-z+utf8::IsWord] (13)
  13: END (0)
stclass ANYOF[0-9A-Z_a-z+utf8::IsWord] minlen 1 

with locale:
Compiling REx "\w"
Final program:
   1: ALNUML (2)
   2: END (0)
stclass ALNUML minlen 1 
Compiling REx "[\w]"
Final program:
   1: ANYOF{loc}[\w+utf8::IsWord] (13)
  13: END (0)
stclass ANYOF{loc}[\w+utf8::IsWord] minlen 1