Multiline pattern match using sed, awk or grep

Question

Is it possible to do a multiline pattern match using sed, awk or grep? Take for example, I would like to get all the lines between { and }

So it should be able to match

 1. {}
 2. {.....}
 3. {.....

     .....}

Initially the question used <p> as an example. Edited the question to use { and}.

Answer 1

While I agree with the advice above, that you'll want to get a parser for anything more than tiny or completely ad-hoc, it is (barely ;-) possible to match multi-line blocks between curly braces with sed.

Here's a debugging version of the sed code

sed -n '/[{]/,/[}]/{
    p
    /[}]/a\
     end of block matching brace

    }' *.txt

Some notes,

• -n means 'no default print lines as processed'.
• 'p' means now print the line.
• The construct /[{]/,/[}]/ is a range expression. It means scan until you find something that matches the first pattern (/[{]/) AND then scan until you find the 2nd pattern (/[}]/) THEN perform whatever actions you find in between the { } in the sed code. In this case 'p' and the debugging code. (not explained here, use it, mod it or take it out as works best for you).

You can remove the /[}]/a\ end of block debugging when you prove to your satisfaction that the code is really matching blocks delimited by {,}.

This code sample will skip over anything not inside a curly brace pair. It will, as noted by others above, be easly confused if you have any extra {,} embedded in strings, reg-exps, etc., OR where the closing brace is the same line, (with thanks to fred.bear)

I hope this helps.

Answer 2

You can use the -M (multiline) option for pcregrep:

pcregrep -M '\{(\s*.*\s*)*\}' test.txt

\s is whitespace (including newlines), so this matches zero or more occurrences of (whitespace followed by .* followed by whitespace), all enclosed in braces.

Update:

This should do the non-greedy matching:

pcregrep -n -M '\{(\n*.*?\n*)*?\}' test.txt

Answer 3

XML like expressions (infinintely recursive tags) is not a 'regular language' therefore cannot be parsed with regular expressions (regex). Here's why:

https://stackoverflow.com/questions/1732348/regex-match-open-tags-except-xhtml-self-contained-tags/

http://www.perlmonks.org/?node_id=668353

https://stackoverflow.com/questions/1379524/textual-protocol-which-is-not-a-regular-language

Answer 4

parser.awk:

#!/usr/bin/awk -f    
function die(msg) { print msg > "/dev/stderr"; exit 1 }
BEGIN {
  FS=opener
  if (mode=="l") linewise=1
  else if (mode=="i") trim_closer=length(closer)
  else if (mode!="a") die("mode must be one of: l,i,a")
}
{
  live=level
  for (f=1; f<=NF; f++) {
    if (f>1) {
      live=++level
      if (mode=="i" && level>1 || mode=="a") printf "%s", opener
    }
    cur=$f
    level-=gsub(closer, "", cur)
    if (level<0) die("Unbalanced")
    if (!linewise) {
      cur=$f
      if (sub(".*" closer, "", cur)) printf "%s", 
        substr($f, 1, length($f) - length(cur) - (level ? 0 : trim_closer))
      else if (live) printf "%s", $f
    }
  }
  if (live) {
    if (linewise) print
    else print ""
  }
}
END { if (level>0) die("Unbalanced") }

Call as awk -v'opener={' -v'closer=}' -v'mode=a' -f parser.awk. If mode is a, it prints the brackets and contents of all outermost, balanced {...}; if mode is i, it prints only their contents; if mode is l, it prints complete lines where an outermost {...} begins, remains open, or closes.

Answer 5

Regular expressions cannot find matching nested parentheses.

If you are certain that there will be no pair of parentheses nested inside the one you are searching, you can search until the first closing one. For example:

sed -r 's#\{([^}])\}#\1#'

This will replace all the text from '{' to '}' with what's between them.