Re: Help needed with substitute* command

[Mortimer Cladwell <mbcladwell@gmail.com>]

Thanks for the detailed explanation Tobias.  Guess I need to brush up on
regular expressions.
Mortimer

On Thu, Jan 6, 2022 at 9:43 AM Tobias Geerinckx-Rice <me@tobias.gr> wrote:

> Hullo Mortimer,
>
> I hope this answer isn't too basic for you.  This input:
>
> Mortimer Cladwell 写道：
> >   ---input.txt(2)-------
> >   foo(abc)bar(def)
>
> does not match the extended regular expression:
>
> > "foo([a-z]+)bar(.*)$"
>
> This would:
>
>   ---input.txt(3)-------
>   foolishbarista
>
> result: bazlishista
>
> I'm not the one to either write or recommend a tutorial on
> extended regular expressions, but you'll find plenty on the 'net.
> There's also ‘info (grep)Regular Expressions’ which might be good.
> These things aren't specific to Guile, although a few dialects
> exist, and I think Guile uses the POSIX one.  The differences are
> quite small.
>
> In this specific example
>
> > ("foo([a-z]+)bar(.*)$" all letters end)
>
> the first string is an extended regular expression.
>
> It will match a literal ‘foo’ anywhere on a line, followed by 1 or
> more lowercase letters, followed by a literal ‘bar’, followed by
> anything until the end of the line.
>
> It will NOT match anything with ‘()’ brackets in it, like your
> original input.txt(2).  The brackets are regexp syntax used for
> grouping and capturing.
>
> If an optional variable name follows the regexp, it will be set to
> the complete match.  Here, that is ‘all’, which in our example
> will contain "foolishbarista".  It's not used here.
>
> In practice, this variable would be named ‘_’ to indicate that
> it's unimportant:
>
>   (("foo([a-z]+)bar(.*)$" _ letters end)
>    (string-append "baz" letters end))
>
> but the author of the manual example thought that ‘all’ would be
> more clear.
>
> Each subsequent optional variable will be set to the content
> matched by () groups.  Here, ‘letters’ will be set to whatever
> matched ‘[a-z]+’, and ‘end’ to whatever matched ‘.*’.
>
> In our example ‘letters’ is "lish" and ‘end’ is "ista".
>
> This is powerful, because we can construct arbitrary strings at
> run time based that can differ significantly for each line that
> matches the same regexp:
>
> > (string-append "baz" letter end)
>
> is just Scheme code that uses the captured variables above,
> without hard-coding assumptions about what was matched.
>
>   footbarnacles → baztnacles
>   foodiebarmaid → bazdiemaid
>   …
>
> Minutes of fun.
>
> This special meaning of ‘()’ in extended rexeps means that if you
> would want to match:
>
>   ---input.txt(4)-------
>   fo(bizzle)
>
> you'd write:
>
>   "fo\\(bizzle\\)"
>
> Because "\" in a string *also* has special meaning to Guile
> itself, we have to write "\\(" if we want the regexp engine to see
> "\(".
>
> >   Is the letters/letter in the manual a typo?  If I use letter I
> >   get
> > "...unbound variable..."
>
> Yes, that was a typo, both names should match.  I've fixed it.
> Thanks for apparently being the first to test this snippet!
>
> Kind regards,
>
> T G-R
>