How to validate TextInput values in react native?

You can use a regex to check if the mail entered is valid.

Regex function

validateEmail = (email) => {
  var re = /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
    return re.test(email);
};

Submit text input function

onSubmit = () => {
if (!this.validateEmail(this.state.text_input_email)) {
  // not a valid email
} else {
  // valid email
}

You can validate your input value using onBlur event on TextInput You can apply your regex or check conditions on this event.

Like this:

<TextInput
      onBlur= () => {
        //Conditions or Regex
      }
/>

In your case, Regex function:

validateEmail = (email) => {
var re = /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
return re.test(email);
};

Text Input Code:

<TextInput
  onBlur= () => {
    if (!this.validateEmail(this.state.text_input_email)) {
       // not a valid email
    } else {
       // valid email
    }
  }
/>