>perl -wMstrict -le
"my $s = 'xyz table 1 table tablet table2 fig 3 fig figer fig4 zyx';
 print qq{'$s'};
 ;;
 $s =~ s{ (?= (?: table | fig) \s* \d+) }{<aid=}xmsg;  # <-- note /g modifier
 print qq{'$s'};
"
'xyz table 1 table tablet table2 fig 3 fig figer fig4 zyx'
'xyz <aid=table 1 table tablet <aid=table2 <aid=fig 3 fig figer <aid=fig4 zyx'