From chang at saitama-med.ac.jp  Fri Aug 15 07:50:43 2008
From: chang at saitama-med.ac.jp (=?ISO-2022-JP?B?GyRCRCUhITRBPSgbKEI=?=)
Date: Fri, 15 Aug 2008 16:50:43 +0900
Subject: [cl-ppcre-devel] Question about a strange behavior of cl-ppcre
Message-ID: <48A53553.9080907@saitama-med.ac.jp>

Hello,

I post this question because the description of this mailing list says
that this is the place to post a question about cl-ppcre.

I am a user of cl-ppcre on sbcl. I just re-installed cl-ppcre on sbcl,
so I guess the version of cl-ppcre is the newest one.

I found a strange behavior as below.

CL-USER> (format t "~A" (cl-ppcre:regex-replace "a" "a" "\\"))
\
CL-USER> (format t "~A" (cl-ppcre:regex-replace "a" "a" "\\\\"))
\
CL-USER> (format t "~A" (cl-ppcre:regex-replace "a" "a" "\\\\\\\\"))
\\

For me, it is very difficult to figure out what's going on. Would
someone kindly help me understand this problem?

Han-Soo



From edi at agharta.de  Fri Aug 15 14:29:49 2008
From: edi at agharta.de (Edi Weitz)
Date: Fri, 15 Aug 2008 16:29:49 +0200
Subject: [cl-ppcre-devel] Question about a strange behavior of cl-ppcre
In-Reply-To: <48A53553.9080907@saitama-med.ac.jp> =?shift_jis?b?KA==?=
	=?shift_jis?b?kqOBQIq/j0cn?=
	=?shift_jis?b?cw==?= message of "Fri, 15 Aug 2008 16:50:43 +0900")
References: <48A53553.9080907@saitama-med.ac.jp>
Message-ID: <u63q2gxlu.fsf@agharta.de>

On Fri, 15 Aug 2008 16:50:43 +0900, ???? <chang at saitama-med.ac.jp> wrote:

> CL-USER> (format t "~A" (cl-ppcre:regex-replace "a" "a" "\\"))
> \
> CL-USER> (format t "~A" (cl-ppcre:regex-replace "a" "a" "\\\\"))
> \
> CL-USER> (format t "~A" (cl-ppcre:regex-replace "a" "a" "\\\\\\\\"))
> \\

The backslash in the replacement specification is special - it can be
followed by things like #\& or #\` to denote specific parts of the
target string - see documentation.  So, if you just want to have a
backslash, you need two backslashes in order to avoid confusion:

  CL-USER 1 > (ppcre:regex-replace "a" "xay" "\\&\\&")
  "xaay"
  T

  CL-USER 2 > (ppcre:regex-replace "a" "xay" "\\&\\\\&")
  "xa\\&y"
  T

Your second example is one (escaped) backslash, your third example
consists of two (escaped) backslashes.  This is conforming with Perl:

  edi at miles:~$ perl -le '$_ = "a"; s/a/\\/; print'
  \
  edi at miles:~$ perl -le '$_ = "a"; s/a/\\\\/; print'
  \\

In your first example, there's only one backslash, but as there's
nothing following it, the parser figured out that you probably meant a
backslash.  This is some kind of a DWIM behaviour and you can of
course argue if it's a good thing or not.

HTH,
Edi.