If you are going to be using it repeatedly, I would place it in a props/transforms. But you can do it via search line.
|rex field=myfield "(?<no_digits>[^\d]*)"
OR with props.conf
[mysourcetype]
EXTRACT-myfield = (?<no_digits>[^\d]*) in myfield
Hello
You could use this regular expresion:
(?<mystring>\D+)
And using the rex command:
... | rex "(?< mystring >\D+)" | ...
*Without the spaces before and after the field name
Regards