Regexp for parsing?

[Wayne Magor]

I understand regular expressions, but can someone please explain this:

re = %r/((?<pg>\((?:\\[()]|[^()]|\g<pg>)*\)))/

By the way, this only works with the Oniguruma engine (Ruby 1.9).

So, now that there is the capability to match balanced parens and so
forth, does this mean that the new regular expression engine can be
used to construct simple parsers (matching language constructs)?

 

[Noah Easterly]

I understand regular expressions, but can someone please explain this:

re = %r/((?<pg>\((?:\\[()]|[^()]|\g<pg>)*\)))/

By the way, this only works with the Oniguruma engine (Ruby 1.9).

So, now that there is the capability to match balanced parens and so
forth, does this mean that the new regular expression engine can be
used to construct simple parsers (matching language constructs)?

%r/ ... /
-- regexp delimter (why they didn't just use / ... /, I don't know)
( ... )
-- non-capturing group - (normally would be capturing, but see
http://www.geocities.jp/kosako3/oniguruma/doc/RE.txt, part 10, case 3)
-- seems rather useless, given that the only contained item is a
capturing group
(?<pg> ... )
-- capturing named group (see http://www.geocities.jp/kosako3/oniguruma/doc/RE.txt,
part 7)
\( .. \)
-- literal parentheses surrounding pattern
(?: ... | ... | ... )*
-- non-capturing group of 3 alternatives, repeated 0 or more times
\\[()]
-- escaped literal parens
[^()]
-- anything except parens
\g<pg>
-- match the pg-named pattern here (recursive sub-exp - see
http://www.geocities.jp/kosako3/oniguruma/doc/RE.txt, part 9)

 

[Wayne Magor]

Thanks, I understand nearly everything now. It really shows the power
of the oniguruma engine for regular expressions. By the way, the comma
caused a Japanese site to come up. For people's reference the manual
for onigurama is at:

http://www.geocities.jp/kosako3/oniguruma/doc/RE.txt

That's a great reference, but I still didn't understand this:

\\[()]
-- escaped literal parens

What is that pattern? I've never seen that before. What does it match?
Where can I read about that?

Why would that be there since a new open paren should start another
instance of <pg>, shouldn't it?

So, obviously, I'm still a little confused.

 

[Eric Hodel]

I understand regular expressions, but can someone please explain this:

re = %r/((?<pg>\((?:\\[()]|[^()]|\g<pg>)*\)))/

By the way, this only works with the Oniguruma engine (Ruby 1.9).

So, now that there is the capability to match balanced parens and so
forth, does this mean that the new regular expression engine can be
used to construct simple parsers (matching language constructs)?

No, translated to 1.8, the regex would be:

%r/((\((?:\\[()]|[^()]|\2)*\)))/

 

[Eric Hodel]

\\[()]
-- escaped literal parens

What is that pattern? I've never seen that before. What does it
match?
Where can I read about that?

"\(" or "\)"

 

[Wayne Magor]

I understand regular expressions, but can someone please explain this:

re = %r/((?<pg>\((?:\\[()]|[^()]|\g<pg>)*\)))/

snip
(?: ... | ... | ... )*
-- non-capturing group of 3 alternatives, repeated 0 or more times
\\[()]
-- escaped literal parens
[^()]
-- anything except parens
\g<pg>
-- match the pg-named pattern here

Ok, so there are 3 alternatives in the non-capturing group:

1. An open or close parenthesis
2. Any character except a paren
3. A pattern that starts with an open paren

Am I the only one that finds this strange?

 

[Noah Easterly]

I understand regular expressions, but can someone please explain this:
re = %r/((?<pg>\((?:\\[()]|[^()]|\g<pg>)*\)))/

snip
(?: ... | ... | ... )*
-- non-capturing group of 3 alternatives, repeated 0 or more times
\\[()]
-- escaped literal parens
[^()]
-- anything except parens
\g<pg>
-- match the pg-named pattern here

Ok, so there are 3 alternatives in the non-capturing group:

1. An open or close parenthesis

correction. As Eric said above, an escaped (read, with leading
backslash) parenthesis.

2. Any character except a paren yup.
3. A pattern that starts with an open paren

AND ends in a close paren, and contains only, non-parens, escaped
parens, and balanced pairs of parens.

Am I the only one that finds this strange?

Doubtful . You may be one of the ones to which this is new, though.

I find it strange that only recognize parenthesis escapes, and not
escaped backslashes. So you can do something like:
( \( )
and match correctly, but there's no way to do a balanced pair of
parentheses containing just a backslash:
(\) -- no
(\\) -- no
(\\\) -- no
(\ ) -- matches, but has an extra space.

I would have replaced '\\[()]' by '\\[()\\]' so that '(\\)' would
match.