how to detect unbalanced special characters in string

Question

I'm wondering what would be the best way (likely using grep or ack) to return lines containing unbalanced special character sets in a string? For example, if the string were:

bqM#+t1U"OyBGhk]ozVG[v"&

and the specified character were a double-quote ("), this line would not be returned. However, what I'm looking for is more complicated, requiring a left and right balance, for instance a left square parenthesis ([) and a right square parenthesis (]). Then the line would be returned if the number of ([) does not equal the number of (]) such as:

i],U2y.2<i*gZCl>y[A~a`^[

Answer 1

With grep implementations that support a -P for perl-like regexps, you could use their recursive regexp abilities to find lines with matching [/] pairs and use -v to reverse that:

grep -Pxv '((\[(?1)\]|[^][])*+)'

Requires the [ to be followed by the corresponding ] i.e [foo] is considered balanced but not ]foo[

Or if the order doesn't matter:

grep -Pxv '((\[(?1)\]|\](?1)\[|[^][])*+)'

[foo] and ]foo[ balanced, [foo still not.

That's in effect returning the lines that don't have the same number of [s as of ]s. A simpler way to achieve that would be to just count them.

perl -lne '$l = () = /\[/g; $r = () = /\]/g; print if $l != $r'

awk 'gsub(/\[/, "&") != gsub(/\]/, "&")'

With sed, you can eliminate pairs in a loop, and print if there are remaining [s or ]s:

sed -e 'h;:1' -e 's/\[\(.*\)\]/\1/g;s/\]\(.*\)\[/\1/g;t1' -e '/[][]/!d;g'

Or if the order matters:

sed -e 'h;:1' -e 's/\[\([^][]*\)\]/\1/g;t1' -e '/[][]/!d;g'