Extract Function

I would encourage you to validate the incoming string before you attempt to use the extract() function on it. There are probably a few ways to do this but in essence if you check that the string you want to process contains an even number of "[" and "]" characters then you'll avoid the error. What might be a bit harder to achieve is to ensure that every "[" has a following "]" without another intervening "["...if that even matters to you? But the principle here: don't attempt to process data that is effectively invalid.

Is there any function to count the number of "[" 

you'll want to check out standard appian expression functions like "find()" to help with that.

edit: find() doesn't actually work for this (grumble) but i believe my example below will.

Not directly, no. You have to break up the string into an array containing its individual characters and then you can count. Here's something that I played with:

a!localVariables(
  local!stringAsArray: fn!char(fn!code(ri!myString)),
  local!scoring: a!forEach(
    items: local!stringAsArray,
    expression: a!match(
      value: fv!item,
      equals: "[",
      then: 1,
      equals: "]",
      then: - 1,
      default: 0
    )
  ),
  local!scoreSum: fn!sum(local!scoring),
  local!statusCode: if(local!scoreSum = 0, 1, 0),
  local!result: if(
    local!scoreSum = 0,
    fn!extract(ri!myString, "[", "]"),
    "INPUT ERROR"
  ),
  a!map(
    statusCode: local!statusCode,
    result: local!result
  )
)

Explanation:

1. fn!char(fn!code(ri!myString)) generates the array of individual characters as described
2. the a!forEach() then assigns a score for each character
3. we then sum the individual scores - if there are an even number of "[" and "]" the score will be zero
4. we then return an a!map() with the result. This contains a statusCode to indicate if the original string was valid or not, and a result

Hope this makes sense

Um...find() only returns the first instance. Frustratingly (since it's such a common use case) there's no function that I'm aware of to count the instances of a string within a string, hence the heavyweight example I've provided.

Um...find() only returns the first instance. Frustratingly (since it's such a common use case) there's no function that I'm aware of to count the instances of a string within a string, hence the heavyweight example I've provided.

yeah, you're right - i realized right after.

so i guess we have to resort to a simple workaround like len(cleanwith(local!myString, "[")) - which will extract only that character from the original string then return the length of the result.

i went through all the functions but couldnt find the exact function to count number of"["

There isn't. So you have to get creative. See above responses.