如何制作允许$（美元符号）在第一个位置的正则表达式？

I am using a ValueConverter to display double types as currency. I want to make sure the entry is valid, so I tried using this validation method I made up. My only problem seems to be that I can't select the whole value (ie 0.00) and type anything. If the '$' is not selected, everything seems fine. How can I fix this? t