$data =~ s/
                    (                     # Capture to $1
                        <a\s              #     <a and a space character
                        (?:               #     Non-capturing parens
                            [^>](?!href)  #         All non > not followed by href
                        )*                #         zero or more of them
                        .?
                        href\s*           #     href followed by zero or more space characters
                    ) 
                    (                     # Capture to $2
                        &\#61;\s*         #     = plus zero or more spaces
                        (                 # Capture to $3
                            &[^;]+;       #     some HTML character code (probably " or ')
                        )?                #     which might not exist
                        (?:               #     Non-grouping parens
                            .(?!\3)       #     any character not followed by $3
                        )+                #     one or more of them
                        .?
                        (?:
                            \3            #     $3 
                        )?                #     (which may not exist)
                   )
                   (                      # Capture to $4
                        [^>]+             #     Everything up to final >
                        >                 #     Final >
                   )
                 /$1 . decode_entities($2) . $4/gsexi;