如果字符串的前4位是0，如何使用Javascript生成警报

I have a form field that users can input a tag number that can only be 9 characters/digits long. The tag number can start with the following: FT followed by seven digits or characters, FM followed by seven characters/digits, or 000 plus 6 characters or digits. If the user inputs four zeros and then 5 digits/characters I need a JavaScript alert to tell them that the tag number cannot start with four zeros. I have tried regular expression by saying:I have a form field that users can input a tag