Regex Question

I have written a generic class that I am using in PHP3 to generate XML statements. Everything is working perfectly except that I want to verify that the variable names that are passed to the class are valid (according to XML rules). Therefore I wish to pass a string of any length through a regex to check its validity, but I have no idea how to do this. The rules are: The FIRST CHARACTER must be an underscore "_" or an alpha character (upper or lower case, no numbers). ALL OTHER CHARACTERS must be alphanumeric OR underscore "_" OR minus "-" OR period "." Can anyone help me to achieve this? ====================================================================== Murray Shields Email: gcat @ bigpond.net.au Global Catalogs Pty Ltd WWW: http://www.globalcat.com.au Brisbane Australia Phone: +61 7 3351 4777 ---------------------------------------------------------------------- Professional Internet Services for Business: Retail Web Catalogs, Web Design, Corporate Email Delivery, Inhouse Web Servers and Consultancy. ======================================================================