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: I wa 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
• No clue to the regular expression pattern expected by PatternExpectation() on the SimpleTest page.
• etc. etc.