Version 12 of Mismatch between regexp -indices and switch -regexp -indexvar

TJE This little mismatch has bitten me in code, so I thought I'd shed a little more light on the matter for those who may not have picked up on it from the documentation...

The ranges placed in a switch statement's "-indexvar" target are inclusive of the character AFTER the match. This differs from the behavior of regexp's "-indices" option, which is exclusive of the same character.

Here's a simple example:

  % set line {foo bar}
  foo bar
  % regexp -inline -indices {foo} $line
  {0 2}
  % switch -regexp -indexvar index -- $line {foo} {set index}
  {0 3}

As you can see, regexp reports the actual match (character '0' through '2' matches "foo"), whereas switch reports the match PLUS the character after (character '0' through '3' matches "foo ").

Don't get bitten!

Curiously, TIP#75 [L1 ] (which seems to be the only one that specifies this feature) states (emphasis added):

the new option -indexvar will also be provided which will name a variable into which a list of match indices (each a two item list of values in the same way that [regexp -indices] computes) will be placed

This rather suggests that the stated mismatch is a bug...

DGP Please have a look at the documentation for regexp and switch. [L2 ] [L3 ]. Appears to me the switch -indexvar option is operating exactly as it is documented to do. As a meta-comment, I think the Tracker is a much better place to resolve question like this than the wiki.

Is it significant that tcl/tests/switch.test does, in fact, have tests for use of -indexvar (in conjuntion with -matchvar) and the tests appear to be passing? Either the tests aren't testing indexvar the way one would think, the test writer cooked the tests so they would pass even though not producing the real expected results, or the code is doing the right thing, but has, perhaps, the wrong docs. See tcl/generic/tclCmdM.c, function Tcl_SwitchObjCmd for the code which provides the switch functionality.

male - 2008-02-15 - for me it is not really interesting, what the man page is telling, if it references the behaviour of regexp, which is different! And even documented behaviour could be buggy! In my eyes both regexp based features should behave the same, no matter what the man page is telling!

set string "The quick brown fox jumped over the lazy dogs."

set matches {}
set indexes {}

switch -regexp -matchvar matches -indexvar indexes -- $string {
  ^(.*)u([a-z]+)(.*)(o[a-z]+)(.*)\.  {
        puts "Found"
        puts " matches = .${matches}."
        puts " indexes = .${indexes}."
   }
  default {
        puts "string = $string"
  }
}

Found
 matches = .{The quick brown fox jumped over the lazy dogs.} {The quick brown fox j} mped { over the lazy d} ogs {}.
 indexes = .{0 46} {0 21} {22 26} {26 42} {42 45} {45 45}.

DKF: Match ranges are the same. Ending index not (one-off). File a bug.
Correction, not a bug. It's documented to be what it is. (Not saying whether it is "morally" right. Just not a bug per se.)

TJE Note that I credit the documents (if indirectly) with correctness in this matter. I don't LIKE the behavior, but it is, indeed, documented. My code is fixed with a weird-looking little '-1' appendage. Here $arg is the switched-upon value and $ipair is the extracted index pair I care about:

  set substring [string range $arg {*}$ipair-1]

Whee!