Php – Regex build to curb user pasting into field

I have users who are pasting strings into text fields that break my calculations. I could really use some help building a Regex expression that:

• only allows numbers 0-9
• only allows one "-" (ascii 45) in the first position in the string
• only allows one "." (ascii 46)
• only allows for up to 3 decimal places after the only instance of "." (ascii 46)