Matching Brackets

Instructions

Given a string containing brackets [], braces {}, parentheses (), or any combination thereof, verify that any and all pairs are matched and nested correctly. Any other characters should be ignored. For example, "{what is (42)}?" is balanced and "[text}" is not.

Tcl is a very simple language, but the way they interpreter parses code has a couple of nasty edge cases.

Curly braces

Braces in Tcl are simply a way to quote a block of text -- that text may be interpreted as code or data. Within braces, nested braces may appear but the nested braces must be balanced. The Tcl man page says this:

Braces nest within the word: for each additional open brace there must be an additional close brace (however, if an open brace or close brace within the word is quoted with a backslash then it is not counted in locating the matching close brace).

The wording there is quite specific: if you want to have an unmatched open or close brace character, it must be quoted with a backslash. So don't do this:

proc isOpenBrace {char} {
    return [expr {$char eq "{"}]
    # ......................^ will not work
}

You must do this

proc isOpenBrace {char} {
    return [expr {$char eq "\{"}]
    # ......................^^ will work
}

This can be problematic with the way Tcl parses comments, which is different from most languages. There is more discussion on the Tcl wiki.

Square brackets

Similarly, since Tcl uses brackets for Command substitution., even within double quoted strings, you have to be careful about escaping open brackets.

Parentheses

With the exception of associative array variables, parentheses are simply ordinary characters.