Check if string is pure ASCII?

Hello all! To add special string-conversions for an imap-client only for strings with special chars I am looking for a function which checks if a string consists only standard ascii-chars. I "see"/googled the following solutions: - check for special chars (i.e. German umlauts) with preg_match() With this solution all other international charsets are not recognized. Not the best solution... - check if string contains only letters and some chars with preg_match() In this case I must add all standard ASCII-chars to regexp. Would be possible. - standard-function? Is there any php-function for this available which I haven't found until now? Thank you for your hints and kind regards, Jens