How to negate specific word in regex? [duplicate]

Answer 1

The accepted answer is nice but is really a work-around for the lack of a simple sub-expression negation operator in regexes. This is why grep --invert-match exits. So in *nixes, you can accomplish the desired result using pipes and a second regex.

grep 'something I want' | grep --invert-match 'but not these ones'

Still a workaround, but maybe easier to remember.

Answer 2

I had a list of file names, and I wanted to exclude certain ones, with this sort of behavior (Ruby):

files = [
  'mydir/states.rb',      # don't match these
  'countries.rb',
  'mydir/states_bkp.rb',  # match these
  'mydir/city_states.rb' 
]
excluded = ['states', 'countries']

# set my_rgx here

result = WankyAPI.filter(files, my_rgx)  # I didn't write WankyAPI...
assert result == ['mydir/city_states.rb', 'mydir/states_bkp.rb']

Here's my solution:

excluded_rgx = excluded.map{|e| e+'\.'}.join('|')
my_rgx = /(^|\/)((?!#{excluded_rgx})[^\.\/]*)\.rb$/

My assumptions for this application:

• The string to be excluded is at the beginning of the input, or immediately following a slash.
• The permitted strings end with .rb.
• Permitted filenames don't have a . character before the .rb.

Answer 3

Just thought of something else that could be done. It's very different from my first answer, as it doesn't use regular expressions, so I decided to make a second answer post.

Use your language of choice's split() method equivalent on the string with the word to negate as the argument for what to split on. An example using Python:

>>> text = 'barbarasdbarbar 1234egb ar bar32 sdfbaraadf'
>>> text.split('bar')
['', '', 'asd', '', ' 1234egb ar ', '32 sdf', 'aadf']

The nice thing about doing it this way, in Python at least (I don't remember if the functionality would be the same in, say, Visual Basic or Java), is that it lets you know indirectly when "bar" was repeated in the string due to the fact that the empty strings between "bar"s are included in the list of results (though the empty string at the beginning is due to there being a "bar" at the beginning of the string). If you don't want that, you can simply remove the empty strings from the list.

Answer 4

Solution:

^(?!.*STRING1|.*STRING2|.*STRING3).*$

xxxxxx OK

xxxSTRING1xxx KO (is whether it is desired)

xxxSTRING2xxx KO (is whether it is desired)

xxxSTRING3xxx KO (is whether it is desired)

Answer 5

Unless performance is of utmost concern, it's often easier just to run your results through a second pass, skipping those that match the words you want to negate.

Regular expressions usually mean you're doing scripting or some sort of low-performance task anyway, so find a solution that is easy to read, easy to understand and easy to maintain.

Answer 6

You could either use a negative look-ahead or look-behind:

^(?!.*?bar).*
^(.(?<!bar))*?$

Or use just basics:

^(?:[^b]+|b(?:$|[^a]|a(?:$|[^r])))*$

These all match anything that does not contain bar.