TCL expression parsing - why brace and bracket are escaped differently

Question

I just did the following experiment in TCL 8.6:

% expr \"\{" ne \"x\"
1

% expr \"\[" ne \"x\"
extra characters after close-quote
in expression ""[" ne "x""

The first command makes sense to me:

Because the argument is not braced, first round parsing is script level parsing, backslash escapes are removed: expr "{" ne "x"
expr command continues the parsing, "{" and "x" are 2 quoted literals and the execution goes well.

The error in the 2nd command does not make sense. The only difference is replacing bracket with brace, why does it fail?

I know bracing the arguments is expected for expression, this question is mostly to understand TCL parsing.

The way that commands are parsed, the Tcl interp just looks for string arguments to pass to the expr command. Your string argument \"[" looks like it starts a string after the [ character but that string is not terminated with a closing double quote. — MoDJ

Donal Fellows Donal Fellows · Accepted Answer · 2016-07-28T14:20:00

The problem with the second command is that the expr command processes […] sequences inside double quotes as command substitutions. This is independent of whether Tcl does and is part of why it is a really good idea to always brace your overall expressions. Had you instead used:

expr \{\[\} ne \"x\"

then it would have worked; just as with the base Tcl language, expr does not expand command substitutions in brace-quoted terms.

TCL expression parsing - why brace and bracket are escaped differently

1 Answers