Bug 57448

I can reproduce this bug. <!--#set var="foo" value="$0" -->  does work, it contains the whole string, but $1 does fail exactly as described earlier.

This bug is still present in 2.4.16 where I have encountered it today.

Example: Show 198. from 198.19.81.98
<!--#if expr="v('REMOTE_ADDR') =~ /(\d+\.)\d+/ && $1 =~ /(\d\.)/" -->
<!--#set var="foo" value="$0" -->
Found <!--#echo var="foo" -->
<!--#endif -->
You *must* use Backreferences in same Expression!

(In reply to Helge from comment #3)
> Example: Show 198. from 198.19.81.98
> <!--#if expr="v('REMOTE_ADDR') =~ /(\d+\.)\d+/ && $1 =~ /(\d\.)/" -->
> <!--#set var="foo" value="$0" -->
> Found <!--#echo var="foo" -->
> <!--#endif -->
> You *must* use Backreferences in same Expression!

That's correct, in the same expression you can use $1 and backreference matches. BUT $0 references always the whole string and not the last matched string. in Helges example 
foo == "198.19.81.98"
and not as expected "198."

if you try use a nested if, you can backreference

<!--#if expr="v('REMOTE_ADDR') =~ /(\d+\.)\d+/ && $1 =~ /(\d\.)/" -->
  <!--# if expr='$1 == "198."' --> 
    Found 198.
  <--#endif -->
<!--#endif -->

it matches. this means backreferences are available in nested ap_expr, but not for any "<!--#set" var or "<!--#echo" operation.

(In reply to Ingmar Eveslage from comment #4)
> (In reply to Helge from comment #3)
> That's correct, in the same expression you can use $1 and backreference
> matches. BUT $0 references always the whole string and not the last matched
> string. in Helges example 
> foo == "198.19.81.98"
> and not as expected "198."

I have tested my Example many times on Apache 2.4.12 an it works as expected: foo returns "198.".
$1 from expression /^(\d\.)/ is $0 for 'set var=foo'.

i boiled it down a litte:

at first: there has to be an indirection with "<!--#set var". echoing directly "$0" doesn't work.

my example shows a work around: 

<!--#set var="test_var" value="1_2_3_4" --> 
<!--#if expr='v("test_var") =~ /(1_)(.*)/ && $1 =~ /(.*)/' --><!--#endif -->
<!--#set var="first" value="$0" -->
<!--#echo encoding='none' var='first' -->

OUTPUT: 1_

changing the second regex in the if statement to $2

<!--#set var="test_var" value="1_2_3_4" --> 
<!--#if expr='v("test_var") =~ /(1_)(.*)/ && $2 =~ /(.*)/' --><!--#endif -->
<!--#set var="first" value="$0" -->
<!--#echo encoding='none' var='first' -->

OUTPUT: 2_3_4

so the simple "$n =~ /(.*)/" acts like an exporter for matched parts.
BUT BE AWARE: don't simplify it "$n =~ /.*/" doesn't work.

i think the bug report stands. somethings doesn't add up.

And why does my RegExample correctly working on my server?
(Last Test on Apache/2.4.12: Fri 2016-02-26 10:50 GMT)

-----------------------
SSILegacyExprParser Off
-----------------------

You CAN use for echoing

#1: #set var="FooBar" value="$0"
    + #echo var="FooBar"

    -OR-

#2: #echo var="0" (var="$0" doesn't work!)


CORRECTED (full) EXAMPLE: Show first ^(\d+\.) from IPv4 Address

<!--#if expr="v('REMOTE_ADDR') =~ /^(\d+\.)/ && $1 =~ /^(\d+\.)/"-->
<!--#set var="FooBar" value="$0" -->
FooBar #1: <!--#echo var="FooBar" --><br>
FooBar #2: <!--#echo var="0" -->
<!--#endif -->

It works!

Thanks for the explanation of the echo part. <!--#echo var="0" --> works.

your example works for me, too. And it does the same as my example. Match a group and match it again in the same expression. so it gets exported as $0.

i think we can agree on that. but i think its still a workaround. $1...$n should be exported directly as gets exported using the legacy parser. Don't you think.

and if the developer do not agree on that, then at least the fact that: 

<!--#if expr='v("test_var") =~ /(1_)(.*)/ && $1 =~ /(.*)/' --><!--#endif -->
<!--#echo var="0" -->
works and 

<!--#if expr='v("test_var") =~ /(1_)(.*)/ && $1 =~ /.*/' --><!--#endif -->
<!--#echo var="0" -->
doesn't, is still a bug. right?

(In reply to Ingmar Eveslage from comment #8)
> Thanks for the explanation of the echo part. <!--#echo var="0" --> works.

I tell you a secret:
I'm personally using 'SSILegacyExprParser On'.
For me and until now It always works. ;-)

Greetings from Helge

good to know. but what are the plans for "SSILegacyExprParser". will it be removed in future versions?

(In reply to Ingmar Eveslage from comment #10)
> good to know. but what are the plans for "SSILegacyExprParser". will it be
> removed in future versions?

I think, it could be removed in Apache/2.5 (?)

 The code shows that $1 is available in the #if, but not #set, whereas $0 is available in the #set.

 <!--#set var="a" value="abc" --> 
<!--#if expr='v("a") =~ /a(b)c/' -->
 <!--#if expr='$1 == "b"' -->
 Got a match.
 <!--#set var="match" value="a$1" --> 
<!--#echo var="match" -->
 <!--#set var="match" value="a$0" --> 
<!--#echo var="match" -->

 =============== 
Got a match.
 a 
aabc

I finally got round to migrating my SSI expressions to the "new" ap_expr syntax, and hit this bug.  And it is clearly a bug.

Because $0 is sometimes (though not always) exported from the "if" to a subsequent "set", you can hack it with extra matchers.  Congratulations to the folks who discovered that, as it's a viable workaround!  But it's clearly a hack, and doesn't work if you want to capture more than one substring.

The documentation for ap_expr suggests that modules can, if they want to, allow the  backref variables to survive between expressions.  It's *partially* happening with SSI (but only with $0, and only if there are capturing parentheses, which in themselves shouldn't affect whether $0 is set), so please can it be fixed to work properly?

I've taken a look at util_expr_eval.c and mod_include.c, and my guess is it's somewhere in the code in parse_ap_expr that decides whether to (re)allocate a backref_t struct within the persistent include_ctx_t.  Hopefully somebody more familiar with this area of code will spot it!

I'm marking this as regression because as initially reported it breaks sites that were working on prior versions.

With the current state of this bug, this is the rigmarole I have to go through simply to impersonate the current directory index header, before I get to customise it with new content:

<!-- Strip the Query-string -->
<!--#if expr='v("REQUEST_URI") =~ /^([^?]*)/ && $1 =~ /(.*)/' -->
<!--#set var="request" value="$0" -->
<!--#else -->
<!--#set var="request" value="${REQUEST_URI}" -->
<!--#endif -->
<!-- strip the final / unless it is the first / -->
<!--#if expr='v("request") =~ /(\x2F.*)\x2F/ && $1 =~ /(.*)/' -->
<!--#set var="request" value="$0" ->
<!--#endif -->
<h1>Index of <!--#echo encoding="entity" var="request" --></h1>

Thanks to Helge & Igmar for showing this work-around.