Grep for words of no more than a certain length

grep '[[:lower:]]*xyz'

Would return you all lines on which that pattern is matched. But, of course, this does not match explicit sequences of characters.

Still, that appears to be a problem you have already solved:

grep -f - <<\STRINGS /dev/fd/3 3<<\DATA
^ZHDEOIMOSJDJKEJLCN
^HDEOIMOSJDJKEJLCN
^DEOIMOSJDJKEJLCN
^EOIMOSJDJKEJLCN
^OIMOSJDJKEJLCN
^IMOSJDJKEJLCN
^MOSJDJKEJLCN
^OSJDJKEJLCN
^SJDJKEJLCN
^JDJKEJLCN
^DJKEJLCN
^JKEJLCN
^KEJLCN
STRINGS

SJDJKEJLCN                                                                  
JDJKEJLCN
o;aidsfjoasjif
KKEJnotLCN
DATA

OUTPUT

SJDJKEJLCN                                                                 
JDJKEJLCN 

If you would like to programmatically generate the same lookup table...

grep -f - <<STRINGS /dev/fd/3 3<<\DATA
$(
    MATCH=ZHDEOIMOSJDJKEJLCN
    until [ ${#MATCH} -lt ${MINLEN=6} ]
    do  printf '^%s\n' "$MATCH"
        MATCH=${MATCH#?}
    done
)
STRINGS

SJDJKEJLCN                                                                  
JDJKEJLCN                                                                  
o;aidsfjoasjif                                                             
KKEJnotLCN                                                                 
DATA

OUTPUT

SJDJKEJLCN                                                                  
JDJKEJLCN

In an attempt to give this Q a proper answer, based - on - the - comments (heeding Sobrique's note that parsing XML should really be done with an XML parser):

perl -CSD -lne 'print for /\w{63,}/g' input-file-here