Re: Question about CL-PPCRE

Isn’t this just Lisp’s natural semantics in strings? If you want a backslash \ in a string, you need to double them up. CL-PPCRE can only see what remains in a string after Lisp has had its go at it. 

This is why Emacs and LW regexps are so injurious to vision.  I find that CL-PPCRE is generally less noxious because it doesn’t depend on backslash for critical regexp elements. 

> On Jun 24, 2026, at 03:07, Martin Simmons <[email protected]> wrote:
> 
>>>>>> On Tue, 23 Jun 2026 20:47:32 +0200, Marco Antoniotti (as marco dot antoniotti at unimib dot it) said:
>> 
>> Hi
>> 
>> I am sure I missed something in the CL-PPCRE documentation, but here it is.
>> 
>> I am fooling around with a C tokenizer and I am running into the following
>> problem.
>> 
>> The string esre contains the "escape sequences" for C (and Perl, etc etc)
>> 
>> CL Prompt > esre
>> "(\\\\(['\\\"\\?\\\\abfnrtv]|[0-7]{1,3}|x[a-fA-F0-9]+))"
>> 
>> If I use CL-PPCRE:REPLACE-ALL I get the following (I tried to boil it down):
>> 
>> CL Prompt > (regex-replace-all "\\$<ES>" "/$<ES>/" esre)
>> "/(\\(['\\\"\\?\\abfnrtv]|[0-7]{1,3}|x[a-fA-F0-9]+))/"
>> T
>> 
>> Apart from the bounding slashes, I would have expected the result of
>> CL-PPCRE:REPLACE-ALL not to return a shorter string.
>> 
>> What exactly (backslash escaping for sure) am I missing?
> 
> It looks like an undocumented feature of the replacement string: \\ is
> converted to a single backslash by CL-PPCRE::BUILD-REPLACEMENT-TEMPLATE.  That
> means it will always return a shorter string when \\ is present in the
> replacement string and if you want \\ in the output then you'll need to use
> \\\\ (i.e. 8 backslashes in the Lisp syntax of that string!).
> 
> Also, to make it more confusing, if \ is followed by anything other than \ & `
> ' digits or {digits} then it is also treated as a single backslash.  i.e. \a
> is \a.
> 
> -- 
> Martin Simmons
> LispWorks Ltd
> http://www.lispworks.com/
> 
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