validation for text field

You can use regexMatch function to make it according to regex. But for than you have to deploy Regex expression plugin in your environment

If you do not want to add a dependency on the plugin, then you can use this code. 

if(
  and(
    a!forEach(
      items: code(ri!name),
      expression: or(
        and(fv!item >= 65, fv!item <= 90),
        and(fv!item >= 97, fv!item <= 122)
      )
    )
  ),
  {},
  "Cannot contain numbers or special characters"
)

you can try this.

if(
contains(
cast(
runtimetypeof(ri!input),
stripwith(
cleanwith(
ri!input,
joinarray(
{
"abcdefghijklmnopqrstuvwxyz",
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
}
)
),
" "
)
),
ri!input
),
"",
"Please use alphabetical characters only."
)