string=<a\n>
   no modifier: regex=/^a$/
     match => $ matches boundary, maybe more?
   s modifier (single line mode): regex=/^a$/s
     match => $ matches boundary, maybe more?
   m modifier (multi line mode): regex=/^a$/m
     match => $ matches boundary, maybe more?
string=<a\n>
   no modifier: regex=/^a$\n/
     match => $ matches only boundary, \n matches newline
   s modifier (single line mode): regex=/^a$\n/s
     match => $ matches only boundary, \n matches newline
   m modifier (multi line mode): regex=/^a$\n/m
     match => $ matches only boundary, \n matches newline
string=<a\n>
   no modifier: regex=/^a$\z/
     no match => $ matches only boundary, \z fails because of newline?
   s modifier (single line mode): regex=/^a$\z/s
     no match => $ matches only boundary, \z fails because of newline?
   m modifier (multi line mode): regex=/^a$\z/m
     no match => $ matches only boundary, \z fails because of newline?
string=<a\n\n>
   no modifier: regex=/^a$/
     no match => $ matches only boundary, \n matches first newline
   s modifier (single line mode): regex=/^a$/s
     no match => $ matches only boundary, \n matches first newline
   m modifier (multi line mode): regex=/^a$/m
     match => $ matches only boundary, \n matches first newline
string=<a\n\n>
   no modifier: regex=/^a$\n/
     no match => $ matches only boundary, \n matches first newline
   s modifier (single line mode): regex=/^a$\n/s
     no match => $ matches only boundary, \n matches first newline
   m modifier (multi line mode): regex=/^a$\n/m
     match => $ matches only boundary, \n matches first newline