Developer Forum

Hello

I tried scanning for multiple occurences of a group in a string and
match/scan would return only one.


"ajabcabck".match /^a*j(?:b*(a+)b+c*)+k$/
=> #<MatchData "ajabcabck" 1:"a">

"ajabcabck".scan /^a*j(?:b*(a+)b+c*)+k$/
=> [["a"]]


clearly the a+ group must match twice to match the string from ^ to $
but only single match is returned.

It is possible to use split instead but using a single match would be
much nicer.

Any workaround?

ruby 1.8.7 (2010-01-10 patchlevel 249) [i486-linux]


Thanks

Michal

Hi
as far as i know, nested groups are not allowed. regular expressions do
not form a language.

regards
ralf

Actually they are allowed, otherwise I would not get a match at all.
Note also that I have manually unnested them in the example. The
problem is that repeated matches of the group are not returned.

Thanks

Michal

Even so, I still think that there is a bug in your regex. I can't
find it, but I tried the same regular expression in perl and in Reggy,
a regex tool for osx ( http://www.yqcomputer.com/ ). Both cases only matched
the one a.

Ben

But the regular expression you're passing is anchored, so the entire
regexp is only matched once, and it only contains one capturing group.

Perhaps this is clearer:

=> [["b", "c"]]
=> [["c"]]

In both cases the result is an array containing a single element,
because the regexp was matched exactly once.

The first gives [$1,$2] because there are two capture groups in its
regexp.

The second gives only [$1] because there is a single capture group. It
happens to have matched multiple times, but you get only the last value
for $1.

If multiple values were inserted into the result, then you wouldn't know
if ["foo","bar","baz"] came from [$1,$2,$3] or [$1,$1,$2] or [$1,$1,$1]
or [$1,$2,$2]
--
Posted via http://www.yqcomputer.com/

Well I think that I understand what the OO is saying, let's break the
match down:

"ajabcabck".match /^a*j(?:b*(a+)b+c*)+k$/

/^a*j/ matches "aj" leaving "abcabck"
/(?:b*(a+)b+c*)+ matches "abcabc" leaving "k"
/k$/ matches "k" and we're done

Now there's a capture group inside that second part a non-capture
group which can (and does in this case repeat).

Since it repeats one might think that there would be one capture for
each repetition, but there isn't. Only the first actually gets
captured.

Here's a simpler example:

/^(a)+$/.match("aa").to_a
=> ["aa", "a"]


Also see http://www.yqcomputer.com/
--
Rick DeNatale

Blog: http://www.yqcomputer.com/
Twitter: http://www.yqcomputer.com/
WWR: http://www.yqcomputer.com/
LinkedIn: http://www.yqcomputer.com/

Thanks for the explanations. As mentioned on the page and also
explained in Brian's reply this is a design limitation of the return
value of the match method. It could return the additional matches but
then the return value would have to be structured differently than it
is now for the result to make sense. As scan most likely uses match
internally or at least returns results consistent with match it shares
the limitation.

So something like split has to be used to slice the string into pieces
where either a shorter non-anchored regex can match repeatedly or only
one match can be found.

The case which causes problems and is not actually well captured by
the example is something like

ab=cd,ef, ...

where the regexes for 'ab', 'cd' and the rest are slightly different,
and so is the interpretation.


Thanks

Michal

I would only use #split if you really want to split the string.
Otherwise please see below.



Nested groups *are* allowed. However, one must understand how group
matching works: for each matching group only at most *one* capture is
recorded:

irb(main):001:0> s="abaab"
=> "abaab"
irb(main):002:0> /(?:(a+)b)+/.match s
=> #<MatchData "abaab" 1:"aa">
irb(main):003:0> md = /(?:(a+)b)+/.match s
=> #<MatchData "abaab" 1:"aa">
irb(main):004:0> md.to_a
=> ["abaab", "aa"]
irb(main):005:0> md[1]
=> "aa"
irb(main):006:0>

As you can see from this 1.9.1 test, it is the *last* match. I cannot
provide an official rationale for this, but one likely reason: The
memory overhead for storing arbitrary amount of matches per group can be
significant. Also, the number of groups is known at compile time of a
regular expression while the number of matches of each group is only
known at match time. This makes it easier to allocate the memory needed
for storing a single capture per group because it can be done when the
regular expression is compiled. Please also note that all regular
expression engines I know handle it that way, i.e. you get at most one
capture per group.

In those cases I usually employ a two level approach:

irb(main):015:0> s = "ajabcaabck"
=> "ajabcaabck"
irb(main):016:0> if /^a*j((?:b*a+b+c*)+)k$/ =~ s
irb(main):017:1> $1.scan(/b*(a+)b+c*/){|m| p m, $1}
irb(main):018:1> end
["a"]
"a"
["aa"]
"aa"
=> "abcaabc"
irb(main):019:0>

Because of the way how #scan works we can do:

irb(main):022:0> if /^a*j((?:b*a+b+c*)+)k$/ =~ s
irb(main):023:1> $1.scan(/b*(a+)b+c*/){|m| p m}
irb(main):024:1> end
["a"]
["aa"]
=> "abcaabc"
irb(main):025:0>


Kind regards

robert

--
remember.guy do |as, often| as.you_can - without end
http://www.yqcomputer.com/

Sorry, I mixed grouping and capturing. Concerning grouping, regexp acts
like a language, but not concerning the capturing and for this reason
you have to make that two level trick. Nested caputring would lead to a
tree of results with bad performance, I guess.

regards
ralf

n 9 February 2010 11:49, Ralf Mueller < XXXX@XXXXX.COM > wrote:
Actually, nested capturing is also supported as you can see from the
examples here. What is not supported is returning multiple matches for
a group that matches multiple times.

Thanks

Michal

Are you sure it matches multiple times? As I mentioned earlier in the
thread, I can't get it to do so.

Ben

(stuff)+ matches multiple stuffs but returns only one.

"stuffstuffstuff".match /^(stuff)+$/
=> #<MatchData "stuffstuffstuff" 1:"stuff">

Still can be nested.

"stuffstuffstuff".match /^(stu(ff))+$/
=> #<MatchData "stuffstuffstuff" 1:"stuff" 2:"ff">

Thanks

Michal