DB<113> print join "|",'a<b1<b2' =~ /(a).+?(?:(c.)|$)/
a|

  DB<114> print join "|",'a<b1<c2' =~ /(a).+?(?:(c.)|$)/
a|c2

  DB<115> print join "|",'a<c1<c2' =~ /(a).+?(?:(c.)|$)/
a|c1