Why is Bash pattern match for ?(*[[:class:]])foobar slow?

Question

I have a text file foobar.txt which is around 10KB, not that long. Yet the following match search command takes about 10 seconds on a high-performance Linux machine.

bash>shopt -s extglob
bash>[[ `cat foobar.txt` == ?(*[[:print:]])foobar ]]

There is no match: all the characters in foobar.txt are printable but there is no string "foobar".

The search should try to match two alternatives, each of them will not match:

"foobar"

that's instantenous

*[[:print:]]foobar

- which should go like this:

should scan the file character by character in one pass, each time, check if the next characters are

[[:print:]]foobar

this should also be fast, no way should take a millisecond per character.

In fact, if I drop ?, that is, do

bash>[[ `cat foobar.txt` == *[[:print:]]foobar ]]

this is instantaneous. But this is simply the second alternative above, without the first.

So why is this so long??

Answer 1

The glob matcher in bash is just not optimized. See, for example, this bug-bash thread, during which bash maintainer Chet Ramey says:

It's not a regexp engine, it's an interpreted-on-the-fly matcher.

Since bash includes a regexp engine as well (use =~ instead of == inside [[ ]]), there's probably not much motivation to do anything about it.

On my machine, the equivalent regexp (^(.*[[:print:]])?foobar$) suffered considerably from locale-aware [[:print:]]; for some reason, that didn't affect the glob matcher. Setting LANG=C made the regexp work fine.

However, for a string that size, I'd use grep.

Answer 2

As others have noted, you're probably better off using grep.

That said, if you wanted to stick with a [[ conditional - combining @konsolebox and @rici's advice - you'd get:

[[ $(<foobar.txt)  =~ (^|[[:print:]])foobar$ ]]

Edit: Regex updated to match the OP's requirements - thanks, @rici.

Generally speaking, it is preferable to use regular expressions for string matching (via the =~ operator, in this case), rather than [globbing] patterns (via the == operator), whose primary purpose is matching file- and folder names.