You could do that making you textInput a controlled component

const [textInputValue, setTextInputValue] = useState('')

  const onTextInputChange = (newText) => {
    //Here you can manipulate the value of the textinput as you want

    setTextInputValue(newText)
  }

  return (
    <View style={styles.container}>
      <TextInput style={styles.textInput} value={textInputValue} onChangeText={onTextInputChange} />
    </View>
  );

consider that with this solution you might experience some flickering issues if the user type faster than react-native is able to update the native view.

Otherwise you can try to implement it natively, for instance here's an example of a custom wrapper for the TextInput component which allows to pass a regex to whitelist allowed characters