Filter non-alpha character with grep/regexp

Hi all,
I am trying to filter out those lines that contain a "non-alpha" character.
An example of my input is the following:

zygnematales    grb
zygocactus    grb
zygocactus_truncatus    plt
zygodactyl_foot    prt
zygoma    prt
zygomatic    prt
zygomatic_arch    prt
zygomatic_bone    prt
zygomatic_process    prt

My desired output is the following:

zygnematales    grb
zygocactus    grb
zygoma    prt
zygomatic    prt

I have tried the following grep:

grep '^-' input 

but it doesn't seem to work.
Any help?

Try:

grep -v "[^ a-z]" input

Or use Character Classes:

grep -v "[[:punct:]]" file