how to automatically replace capturing groups

Question

let's say I have a source file that matches a certain filepattern, and I want certain elements of the file pattern to be reused in a newly created destination file.
so for ex. the specified file pattern given in regex is

src_pattern = "(\d\d)_(\d\d)_(\d\d\d\d)_FOO.xml";

after searching a directory, found a matching source file looking like 

src = 08_21_2013_foo.xml

now the new file must be of the form 

dst = $3$1$2_BAR.xml;

where the capturing groups pulled from the source (so would look like 20130821_BAR.xml). How would I go about accomplishing this efficienly, needs to pretty flexible and I have no knowledge of what each of these look like, they are being pulled from somewhere else. so I guess I'm having trouble with pulling the numberings for the capturing groups, ie the 3rd, the 1st, then the 2nd and let's say I found it, how do I reference it back to the source file. would I have to have an integer (say k) that represents the number and reference it like

match = Regex.Match(src, src_pattern)
match.Groups[k].Value

pulling these numberings seem to be a pain...

I also have no idea how many of these capturing groups each dst specifies so how to automate all of this? Is there another way or some smart native functonality for this

xanatos · Accepted Answer · 2013-08-21 13:55:02Z

1

Try with

var rx = new Regex(@"^(\d\d)_(\d\d)_(\d\d\d\d)(?=_FOO\.xml$)", RegexOptions.IgnoreCase);
var res = rx.Replace("08_21_2013_foo.xml", "$3$1$2");

Note the RegexOptions.IgnoreCase, the use of the ^ and $ to force the regex to consider the whole string and the (?=_FOO.xml$) that means "followed by _FOO(end of the string)", but that isn't a capturing group.

answered Aug 21, 2013 at 13:55

xanatos

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1 Comment

Ibrahim Najjar Over a year ago

You forgot _BAR at the end of the file name.

wilso132 · Accepted Answer · 2013-08-21 13:56:55Z

1

If you could get them to use named groups (http://msdn.microsoft.com/en-us/library/bs2twtah.aspx#named_matched_subexpression), you could easily just run a substitute based on whatever they requested and have them name them again in the resulting output dst. For example:

src_pattern = "(<first>\d\d)_(<second>\d\d)_(<third>\d\d\d\d)_FOO.xml";

after searching a directory, found a matching source file looking like 

src = 08_21_2013_foo.xml

now the new file must be of the form 

dst = "[third][first][second]_BAR.xml";

answered Aug 21, 2013 at 13:56

wilso132

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3 Comments

Luke Makk Over a year ago

it's all specified and numbered groups instead of named groups. the most important part is I do not know beforehand I want the third, the first, then the second so can't hardcode these.

wilso132 Over a year ago

You need to expand upon your question then... if you don't know beforehand, at what point will you know? From your example above it appeared these settings are config file driven.

Luke Makk Over a year ago

I said it's coming from a different source, a data file or sql perhaps but there are thousands if not more, I don't know what the src and dst look, that was just one specific example.

Maslow · Accepted Answer · 2013-08-21 16:20:58Z

1

So we take the regex and provide some sample data.
for each match in mockMatches which you would replace with the enumerator that pulls in your list of filenames.
we then match the regex using the src_pattern with ignore case turned on.
then we want the matching groups which comes out as a GroupCollection so we Cast<T>() that Enumerable as IEnumerable<Group>
skip the first group (which is the entire match) using Skip(1)
then get the group's value (the actual text of the group match) with .Select(a=>a.Value)
join all of those together using a _ as the seperator with .Aggregate((s1,s2)=>s1+"_"+s2)
then add the file ending constant "_bar.xml"

Linqpad based answer:

var src_pattern= @"(\d\d)_(\d\d)_(\d\d\d\d)_FOO\.xml";
var mockMatches = new[]{"08_21_2013_foo.xml"};
foreach(var mm in mockMatches){
  var match = Regex.Match(mm,src_pattern, RegexOptions.IgnoreCase).Dump();
  var dst= match.Groups.Cast<Group>().Skip(1).Select(a=>a.Value).Aggregate((s1,s2)=>s1+"_"+s2)+"_bar.xml";
  dst.Dump();
}

edited Aug 21, 2013 at 16:20

answered Aug 21, 2013 at 13:56

Maslow

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4 Comments

Luke Makk Over a year ago

what is cast and dump, sorry never used linqpad

Maslow Over a year ago

Cast<T> is a Linq method, Dump is kinda like console.writeline but WAY better.

Luke Makk Over a year ago

can you roughly explain the logic

Luke Makk Over a year ago

i really appreciate great solution but I have a more general question in a larger context really

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