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 docs page is not particularly helpful in explaining expected syntax.
• Nor can I easily work out the syntax expected here if i just land on the page from a search.
• No explicit mention of the regular expression pattern expected by PatternExpectation() on the SimpleTest page.

• etc. etc.

• here is a highly voted SO answer that seems to assume there is only one flavour of regular expressions.