I need help validating " First Name" input field. The validation is the field should not accept numbers and special characters. There is no button at this point . But message should pop as user tries to enter data with numbers and special characters.In other words it should only accept characters.
You just need to specify a pattern:
<lightning:input ... pattern="[a-zA-Z]*" messageWhenPatternMismatch="Please use only letters." />
If you're not familiar with regular expressions, you can read about them here (or other resources).