Documentation often ommits to specify which flavour of regular expression to use, so is there a default flavour that we should all be familiar with?

Question

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.

Answer 1

I suspect Documentation does not specify which flavour of regular expression to use, because it's rarely a choice. The choice is usually specified by the language that has been chosen (unless the language implements more than one flavour, in which case it might be valid to specifiy a preference for one of them in the docs, to keep things consistent), or by other constraints of the programming environment.

As a handy reference, here's a web page comparing regex flavours: http://www.regular-expressions.info/refflavors.html

Answer 2

I usually go with the flavor found in books dealing with Perl. Rarely fails.