# the lack of a $var =~ here indicates
              # that we are matching against $_
m             # this is a match regex
!             # we define the extent of the regex using a !
^             # starting at the begining of the string
(             # grouping and capturing opening bracket
<             # matches this charachter '<'
[A-Za-z0-9]+  # match any number of alphanumerics in a row
>             # match a literal '>'
)             # closing bracket
              # the match between brackets captures to $1
*             # accept 0 or more of the preceeding stuff
              # but note only first match captured into $1
              # within the brackets
<H1>          # match a literal '<H1>'

(<[A-Za-z0-9]+>)* # same as first but captures into $2
                  # the first '<alphanumeric>' sequence
([A-Za-z0-9]+)    # matches sequential alphanumerics and
                  # captures all into $3
!                 # end of regex