grep -P

tomazos has asked for the wisdom of the Perl Monks concerning the following question:

In the man page for grep it says:

 -P, --perl-regexp
              Interpret PATTERN as a Perl regular expression.
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When I try this I get:

$ grep -P 'whatever'
grep: The -P option is not supported
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(Using Ubuntu 6.06 Dapper Drake)

Any ideas on the history of this feature? Why is it "not supported"? How do I get a version of grep that supports it?

When the feature works, what is the extent of greps support for Perl regexps? Does it actually use perl, or is it a reimplemented engine?

-Andrew.