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. 

But, at my last place of work I was made to feel stupid when I suggested adding some text to our User Documentation to specify *the type* of regular expression to be used.

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

Update: I was prompted to single-out some examples - but no disrespect to these great projects:

* [eregi](http://php.net/manual/en/function.eregi.php) docs page is not particularly helpful in explaining expected syntax.
* Nor can I easily work out the syntax expected [here](http://www.fileformat.info/tool/regex.htm) if i just land on the page from [a search](http://www.google.co.uk/search?hl=&q=regular%20expression%20test).
* No explicit mention of the regular expression pattern expected by `PatternExpectation()` on the [SimpleTest page](http://www.simpletest.org/en/expectation_documentation.html).
* etc. etc.
* here is a highly voted [SO answer that seems to assume there is only one flavour of regular expressions](https://stackoverflow.com/a/2841557).