$
$ cat -n data.txt
     1  field 12345 12345 a
     2  field 12345 a
     3  field 12345 123456 a
     4  field 12345
     5  field 12345 12aQ~
     6  field 12345 12aQ~ 123
     7  field 12345 1340 4578 abc AB#xy2
$
$

$
$ perl -ne '@a=split; $f=$a[2]; ($x=$f)=~s/\d//g; ($y=$f)=~s/\D//g;
>           print if ( $#a<2 or
>                      ($#a>=2 and $x eq $f) or
>                      ($#a>=2 and $y eq $f and length($f)<=5)
>                    )' data.txt
field 12345 12345 a
field 12345 a
field 12345
field 12345 1340 4578 abc AB#xy2
$
$