Re: regexps

Monday, 3 January 2011

        Jamie Zawinski writes:

...
 So,  "#r" can mean "just do backslash hacking",
as now, for
 historical compatibility, and the new "#R" can mean "read this in
 Perl syntax". 
...
 - return Funcall ("perl-regexp-to-emacs-regexp",
regexp_string, regexp_flags); 
...
 That function can either mechanically transform the string into an
 Emacs-syntax string, 
OK.  I still don't like the reader macro enough to implement it
myself, though.

I do think we should implement `perl-regexp-to-emacs-regexp'.  Even
using my own copious free time.

...
 or it can return a new object of with type-of "perl-regexp"
which
 prints itself with #R syntax and which string-match accepts as an
 argument.  (Maybe perl-regexp objects can also be typep 'string.) 
That, I think, is a bad idea.  I think that users would likely to be
annoyed whether (concat #R(a)...＠ "...") returns an ordinary string that
is interpreted as an Emacs-syntax regexp when handed to a matching
function, or a perl-regexp object, or a "non-regexp-string" object,
depending on which is the user-supplied part.

...
 > And what about #r/foo\bar/unknown -- would that be a syntax
error?
 > But then you'd need to embed knowledge of the flag syntax in the lexer.

 #'perl-regexp-to-emacs-regexp just signals, resulting in a parse
 error. 
OK.

...
 Incidentally, #r is inconsistent, as it has no way to read a string
 ending in a backslash or a double-quote.  That is, the rules for
 parsing character N differ from the rules for characters [0-N).  A
 more consistent grammar would prohibit \" within #r, which is
 exactly why sed-syntax regexps allow arbitrary quote-characters
 after the "s". 
Sure.  I've only needed such strings as examples of what not to do,
though, so I don't consider them a big loss.

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