m/
   ^                  # start of line
     1?               # the number 1, zero or one time
   $                  # end of line
 
   |                  # alternation
 
   ^                  # start of line
     (                # remember the match in \1 
       11+?           # the number 1, then the number 1 once or more times, but the less time possible
     )                # 
     \1+              # the matched sequence, once or more.
   $
/x ;