How to match a pattern in lines before another pattern match

Question

This question is similar to How to show lines after each grep match until other specific match?

I want to match a particular pattern in lines before another pattern match.

Here I want to get the file from a given host. Each file can have multiple hosts. Hence I don't have a fix number of lines before I get to the hostfile tag from a given host.

Context:

...
...
<hostfile file:abc.txt>
   <host> abc.com <\host>
   <host> qwe.com <\host>
   <host> xyz.com <\host>
<\hostfile>
...
<hostfile file:xyz.txt>
   <host> asd.com <\host>
<\hostfile>
...
...

Example match

Input: xyz.com 
Output: abc.txt

Input: asd.com
Output: xyz.txt

Using awk or sed or any other command-line tool.

Answer 1

Another awk variation:

/^<hostfile file:/ {
        output=substr($2, 6, index($2, ">") - 6);
}
/<host>/ && $0 ~ pattern {
        print output
}

Call it such as:

$ awk -v pattern='xyz.com' -f findit.awk contextfile
abc.txt
$ awk -v pattern='asd.com' -f findit.awk contextfile
xyz.txt

Answer 2

You can't parse XML with regex. Because XML can't be parsed by regex. Regex is not a tool that can be used to correctly parse XML. As I have answered in XML-and-regex questions here so many times before, the use of regex will not allow you to consume XML. Regular expressions are a tool that is insufficiently sophisticated to understand the constructs employed by XML. XML is not a regular language and hence cannot be parsed by regular expressions. Regex queries are not equipped to break down XML into its meaningful parts. so many times but it is not getting to me. Even enhanced irregular regular expressions as used by Perl are not up to the task of parsing XML. You will never make me crack. XML is a language of sufficient complexity that it cannot be parsed by regular expressions. Even Jon Skeet cannot parse XML using regular expressions. Every time you attempt to parse XML with regular expressions, the unholy child weeps the blood of virgins, and Russian hackers pwn your webapp. Parsing XML with regex summons tainted souls into the realm of the living. XML and regex go together like love, marriage, and ritual infanticide. The <center> cannot hold it is too late. The force of regex and XML together in the same conceptual space will destroy your mind like so much watery putty. If you parse XML with regex you are giving in to Them and their blasphemous ways which doom us all to inhuman toil for the One whose Name cannot be expressed in the Basic Multilingual Plane, he comes. XML-plus-regexp will liquify the nerves of the sentient whilst you observe, your psyche withering in the onslaught of horror. Rege̿̔̉x-based XML parsers are the cancer that is killing StackOverflow it is too late it is too late we cannot be saved the trangession of a chi͡ld ensures regex will consume all living tissue (except for XML which it cannot, as previously prophesied) dear lord help us how can anyone survive this scourge using regex to parse XML has doomed humanity to an eternity of dread torture and security holes using regex as a tool to process XML establishes a breach between this world and the dread realm of c͒ͪo͛ͫrrupt entities (like SGML entities, but more corrupt) a mere glimpse of the world of regex parsers for XML will instantly transport a programmer's consciousness into a world of ceaseless screaming, he comes, the pestilent slithy regex-infection will devour your HTML parser, application and existence for all time like Visual Basic only worse he comes he comes do not fight he com̡e̶s, ̕h̵is un̨ho͞ly radiańcé destro҉ying all enli̍̈́̂̈́ghtenment, XML tags lea͠ki̧n͘g fr̶ǫm ̡yo​͟ur eye͢s̸ ̛l̕ik͏e liquid pain, the song of re̸gular expression parsing will extinguish the voices of mortal man from the sphere I can see it can you see ̲͚̖͔̙î̩́t̲͎̩̱͔́̋̀ it is beautiful the final snuffing of the lies of Man ALL IS LOŚ͖̩͇̗̪̏̈́T ALL IS LOST the pon̷y he comes he c̶̮omes he comes the ichor permeates all MY FACE MY FACE ᵒh god no NO NOO̼OO NΘ stop the an*̶͑̾̾​̅ͫ͏̙̤g͇̫͛͆̾ͫ̑͆l͖͉̗̩̳̟̍ͫͥͨe̠̅s ͎a̧͈͖r̽̾̈́͒͑e not rè̑ͧ̌aͨl̘̝̙̃ͤ͂̾̆ ZA̡͊͠͝LGΌ ISͮ̂҉̯͈͕̹̘̱ TO͇̹̺ͅƝ̴ȳ̳ TH̘Ë͖́̉ ͠P̯͍̭O̚N̐Y̡ H̸̡̪̯ͨ͊̽̅̾̎Ȩ̬̩̾͛ͪ̈́̀́͘ ̶̧̨̱̹̭̯ͧ̾ͬC̷̙̲̝͖ͭ̏ͥͮ͟Oͮ͏̮̪̝͍M̲̖͊̒ͪͩͬ̚̚͜Ȇ̴̟̟͙̞ͩ͌͝S̨̥̫͎̭ͯ̿̔̀ͅ

Answer 3

I wanted to apply some XML parsers but the presented "Context" doesn't fit a valid XML structure at all (invalid attributes file:abc.txt, invalid closing tags <\host>, <\hostfile>).
So here's gawk "hack":

awk  'BEGIN{ RS="<hostfile" }/file:/{ f = substr($1,6,length($1)-6) }
      /<host>/{ match($0,/.*<host>([^<>]+)<\\host>[[:space:]]*<\\hostfile>.*$/,a);  
      printf("Input: %s\nOutput: %s\n\n",a[1],f)}' yourfile

The output:

Input:  xyz.com 
Output: abc.txt

Input:  asd.com 
Output: xyz.txt