how to mak Infopath form textbox allow only alphabets

I am trying to create a textbox in info path form with following criteria:

1.) Length should be 1-50 characters .

2.) It should not contain any digit.

3.) It should contain only alphabets.

I try following pattern but it's not working

FieldName | does not math pattern | [a-zA-Z]{1,50} | or
FieldName | does not math pattern | .*[a-zA-Z].*   | 

following regex pattern work with allow special character but not work in Infopath form

^[^0-9]+$