[racket-dev] Missing pregexp syntax in Racket

From: David T. Pierson (dtp at mindstory.com)
Date: Mon Nov 28 00:48:12 EST 2011

On Sat, Nov 26, 2011 at 10:35:57PM -0800, Pauan wrote:
> It was brought up that my explanation was confusing, and I agree it is.
> So I'll try again. The following should return #t:
> (regexp-match? "\\n"        "\n")

I am confused about a number of things in your emails, so for simplicity
I'm focusing on the above expression only.

Your email subject mentions pregexp, but your pattern is a string
literal, which AFAICT will be compiled into a regular expression using
regexp not pregexp.  Therefore it isn't clear whether you are suggesting
a change to regexp syntax or pregexp syntax.

Currently, #rx"\\n" matches like #rx"n" and #px"\\n" is an error:

> (regexp-match? #rx"\\n" "n")
> #px"\\n"
readline-input::569: read: bad pregexp string: illegal alphabetic escape

Both of these behaviors agree with the syntax documented at


To add to my confusion, your original email mentioned #px"\\\\n", which
currently matches a backslash followed by an 'n'.

Perhaps you are suggesting that #px"\\n" should mean the same as
#px"\n" rather than being an error?  I don't see a need for this but
perhaps you have a rationale in mind?


Posted on the dev mailing list.