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Re: Transposing Regular Expression


From: Andreas Politz
Subject: Re: Transposing Regular Expression
Date: Wed, 11 Nov 2009 22:45:17 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)

jrwats <address@hidden> writes:

> Perl provides the transpose operator:
> =~ tr/abc/xyz/        not really a regular expression, but exchanges 'x' for
> 'a', 'y' for 'b', and 'z' for 'c' in the source string.
>
> My question is how to accomplish this in emacs.  When only needing to
> tranpose 2 characters that need to replace each other, (the equivalent
> perl expression woud be =~ tr/ab/ba/ as an example, I could simply
> regexp replace 'a' with a unique letter or symbol, maybe '$' for
> instance, then replace all b's with a's and all $'s with b's.  This
> obviously becomes unweildy after we start transposing more than 2
> characters.  My question is, now that emacs provides fancy regexp
> replace clauses: \# for the number match, and arbitrary lisp
> expressions \,(some-lisp), etc, is there a way to accomplish this in
> one fell swoop via a very crazy regular expression find-replace?  Also
> is there a list of meaningful  regular expression search escape
> characters somewhere (like \#) ?

You can use literal backreferences (e.g. \1) in the replacement part of
interactive uses of replace-regexp.  Combine that with expressions \,
and you could write a c-compiler in emacs-regexp/elisp.

The regular expression syntax is described in the elisp manual.

(info "(elisp)Regular Expressions")

-ap





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