AWK negative regular expression

Question

I am looking to have a awk regular expression that can give all strings not matching a particular word.

using /^((?!word \+).)*/ works in java but does not work in AWK.

Get compilation failed error, escaping the brackets fixes the compilation error, but the regular expression matching is not correct.

It would be great if any one can help with a awk regular expression .

I can not use string" !~ /regex/

I need to use string" ~ /regex/ regex shuould pass for all string but for a specific string.

Strings containing domain should be filtered out. Input

This is domain test
This is do test
This is test

Output

This is do test
This is test

Need to do with regular expression only. Can not change the Awk code

in AWK its like string" ~ /regex/

so can only pass a regex to achieve this.

Answer 1

While Thomas Dickey's answer is clever, there is a right way to do this:

awk '!/domain/ {print}' <<EOF
This is domain test
This is do test
This is test
EOF

This is do test
This is test

Answer 2

The way you do this in awk is to write a pattern that matches what you want to exclude, and make its action next. That leaves the lines to process which are the negation of the pattern which you are trying to achieve.

Something like this:

/regex/{ next; }
{ print; }

Or if your script is more complicated and you can't use next:

<<<'the quick brown fox jumps over the lazy dog'$'\n''pack my box with five dozen liquor jugs' awk '
{ negate=0 }
/box/{ negate=1 }
negate==1 { gsub(/[aeiou]/, "%") }
negate==0 { gsub(/[aeiou]/, "#") }
{ print NR, $0; }
'

# output
1 th# q##ck br#wn f#x j#mps #v#r th# l#zy d#g
2 p%ck my b%x w%th f%v% d%z%n l%q%%r j%gs

Answer 3

Because you stated "Need to do with regular expression only. Can not change the Awk code" try this:

Instead of "negation of the operator" you can "negate the regex". You just need to pass negation symbol ! as part of regex !/domain/ to awk script.

awk '{if ($0 ~ !/domain/) {print $0}}'

Answer 4

awk does not support PCRE (Perl Compatible Regular Expression), so you can not use any zero width lookarounds like the negative lookahead you are using, (?!word +).

awk's Regex engine only supports ERE (Extended Regular Expression). You need to incorporate all logics in there or you can break up the logic to meet your need. Also look at the builtin functions of awk, check if they are of any help in the regard.

Answer 5

Writing a negated pattern with POSIX extended regular expressions is particularly tricky. For this case of a string like domain where all the characters are different, you can try:

awk -v re="^([^d]|d*d[^do]|[do]*o(d*d[^do]|[^dm])|[dom]*m(d*d[^do]|[do]\
*o(d*d[^do]|[^dm])|[^da])|[doma]*a(d*d[^do]|[do]*o(d*d[^do]|[^dm])|[dom\
]*m(d*d[^do]|[do]*o(d*d[^do]|[^dm])|[^da])|[^di])|[domai]*i(d*d[^do]|[d\
o]*o(d*d[^do]|[^dm])|[dom]*m(d*d[^do]|[do]*o(d*d[^do]|[^dm])|[^da])|[do\
ma]*a(d*d[^do]|[do]*o(d*d[^do]|[^dm])|[dom]*m(d*d[^do]|[do]*o(d*d[^do]|\
[^dm])|[^da])|[^di])|[^dn]))*[domai]*\$" '$0 ~ re'

I did come up with that years ago and come back to that article of mine whenever I need to write a negative regexp, but I must admit I don't remember how I got to that.

Answer 6

Tried with below 3 methods

First method using awk

• command

  awk '$0 !~/domain/{print $0}' file.txt
  

  resulting output

  This is do test
  This is test
  

Second method using sed

• command

  sed -n '/domain/!p' file.txt
  

• resulting output:

  This is do test
  This is test
  

Third method using python

• python script

  #!/usr/bin/python
  k=open('test.txt','r')
  for i in k:
      if "domain" not in i:
          print i.strip()
  

• resulting output

  This is do test
  This is test