Often I come across documentation that says "use a regular expression here" I have to spend quite some time digging around trying to work out *which regular expression* format they are expecting. 

As far as I can tell, there are many types of regular expression.

When someone says "a regular expression" what is *the* regular expression syntax most people expect and where is it documented?

Update: Example [eregi][1] - is not particularly helpful in syntax.


  [1]: http://php.net/manual/en/function.eregi.php