Preg Match Empty String

Question

How to preg_match for an null (empty) string??

I need something like:

/([0-9]|[NULL STRING])/

Answer 1

You can use ^ to match beginning-of-line, and $ to match end-of-line, thus the regular expression ^$ would match an empty line/string.

Your specific example (a line/string containing a single digit or being empty) would easiest be achieved with ^[0-9]?$.