Grep ambiguity nested square bracket

grep, regex

Solution

First thing to understand is, inside a character class, none of the meta-characters of regex has any special meaning. They are matched literally. For e.g., an `*` will match a `*` and will not mean `0 or 1` repetition. Similarly, `()` will match `(` and `)`, and will not create a `capture group`.

Now, if a `]` is found in a character class, that automatically closes the character class, and the further character won't be the part of that character class. Now, let's understand what is happening above:

In `1`, `2`, and `4`, your character class ends at the first closing `]`. So, the last closing bracket - `]`, is not the part of character class. It has to be matched separately. So, your pattern will match something like this:

'[[ab]]' is same as '([|a|b)(])'  // The last `]` has to match.
'[ab[]]' is same as '(a|b|[)(])'  // Again, the last `]` has to match.
'[ab]]'  is same as '(a|b|])(])'  // Same, the last `]` has to match.
    ^
    ^---- Character class closes here.

Now, since in both the string, there is no `]` at the end, hence no match is found.

Whereas, in the 3rd pattern, your character class is closed only by the last `]`. And hence everything comes inside the character class.

'[ab[]' means match string that contains 'a', or 'b', or '['

which is perfectly valid and match both the string.

And what does `[(ab)]` and `[^(ab)]` mean?

`[(ab)]` means match any of the `(`, `a`, `b`, `)`. Remember, inside a character class, no meta-character of regex has any special meaning. So, you can't create groups inside a character class.

`[^(ab)]` means exact opposite of `[(ab)]`. It matches any string which does not contain any of those characters specified.

Is it the same as `[ab]` and `[^ab]` ?

No. These two does not include `(` and `)`. Hence they are little different.

Problem

Original source