This is the portion of gdb_test which performs the match against
the RE (regular expression) passed to it:
return [gdb_test_multiple $command $message {
-re "\[\r\n\]*($pattern)\[\r\n\]+$gdb_prompt $" {
if ![string match "" $message] then {
pass "$message"
}
}
In a test that I've been working on recently, I wanted to use
a backreference - that's the \1 in the the RE below:
gdb_test "info threads" \
{.*[\r\n]+\* +([0-9]+) +Thread[^\r\n]* do_something \(n=\1\) at.*}
Put into English, I wanted to make sure that the value of n passed to
do_something() is the same as the thread number shown in the "info
threads" Id column. (I've structured the test case so that this
*should* be the case.)
It didn't work though. It turned out that ($pattern) in the RE
noted above is capturing the attempted backreference. So, in this
case, the backreference does not refer to ([0-9]+) as intended, but
instead refers to ($pattern). This is wrong because it's not what I
intended, but is also wrong because, if allowed, it could only match a
string of infinite length.
This problem can be fixed by using parens for a "noncapturing
subpattern". The way that this is done, syntactically, is to use
(?:$pattern) instead of ($pattern).
My research shows that this feature has been present since tcl8.1 which
was released in 1999.
The current tcl version is 8.6 - at least that's what I have on my
machine. It appears to me that mingw uses some subversion of tcl8.4
which will also have this feature (since 8.4 > 8.1).
So it seems to me that any platform upon which we might wish to test
GDB will have a version of tcl which has this feature. That being the
case, my hope is that there won't be any objections to its use.
When I looked at the implementation of gdb_test, I wondered whether
the parens were needed at all. I've concluded that they are. In the
event that $pattern is an RE which uses alternation at the top level,
e.g. a|b, we need to make $pattern a subpattern (via parens) to limit
the extend of the alternation. I.e, we don't want the alternation to
extend to the other portions of the RE which gdb_test uses to match
potential blank lines at the beginning of the pattern or the gdb
prompt at the end.
gdb/testsuite/ChangeLog:
* gdb.exp (gdb_test): Using noncapturing parens for the $pattern
subpattern.
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