In QML a RegExpValidator is not applied right after TextInput component is created. As QRegExpValidator will accept anything by default, TextInput.acceptableInput will return true when the component is created. Once TextInput value is modified, the right regexp is applied.

The following code will show the issue. When the application is started the acceptableInput will be true thus the color is set to green.

import QtQuick 1.0
Rectangle {
    id: root
    color: input.acceptableInput ? "green" : "red"
    width: 300; height: 300
    Rectangle {
        anchors.centerIn: parent
        height: 50
        width: 50
        color: "white"
        TextInput {
            id: input
            anchors.fill: parent
            validator: RegExpValidator { regExp: /.{1,4}/ } 
        }
    }
}