Debugging regular expressions

       There are two ways to enable debugging output for regular
       expressions.
 
       If your perl is compiled with -DDEBUGGING, you may use the
       -Dr flag on the command line.
 
       Otherwise, one can use re 'debug', which has effects both
       at compile time, and at run time (and is not lexically
       scoped).

       [snip; read the details in the perldebug documentation.]