[cl-ppcre-devel] Question about a strange behavior of cl-ppcre
張 漢秀
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
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Edi Weitz
> 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@miles:~$ perl -le '$_ = "a"; s/a/\\/; print'
\
edi@miles:~$ 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.