DB<1> $s = 123

  DB<2> x $s =~ /(\d)/g
0  1
1  2
2  3
  DB<3> x $s =~ /([\d])/g
0  1
1  2
2  3
  DB<4> x $s =~ /([:digit:])/g
  empty array
  DB<5> x $s =~ /([[:digit:]])/g
0  1
1  2
2  3