Re: Help needed with substitute* command

[Tobias Geerinckx-Rice <me@tobias.gr>]

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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

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