[SR-Users] Understanding regex format for subst_hf

[Daniel-Constantin Mierla]

Hello,

the regular expressions in textops have to be POSIX format (extended,
iirc), the remark for perl/sed-like was referring to the format being
'/match/replace/flags' that is common for Perl scripting and sed parameters.

Practically textops uses the regex functions from libc, does not depend
on libpcre that offers PCRE format for expressions.

The regex supports matching with PCRE and dialplan offers a way to do
substitutions by matching with PCRE library.

I updated the readme of textops to reflect somehow better the above.

Cheers,
Daniel

On 04.10.21 19:50, Joel Serrano wrote:
> So now when combined with subst_hf to actually edit the header value,
> I'm getting this error:
>
> Oct  4 10:44:44 kamailio-n1 kamailio[20324]: ERROR: <core>
> [core/re.c:363]: replace_build(): Buffer too small
> Oct  4 10:44:44 kamailio-n1 kamailio[20324]: ERROR: textops
> [textops.c:3110]: subst_hf_f(): textops subst_run failed
>
> Can anyone remind me the name of the parameter I have to adjust to
> increase the buffer size?
>
> Thanks, 
> Joel.
>
> On Mon, Oct 4, 2021 at 10:46 AM Joel Serrano <joel at textplus.com
> <mailto:joel at textplus.com>> wrote:
>
>     Update:
>
>     I managed to achieve it using the "regex" module with
>     "pcre_match()", that will take negative lookahead nicely.
>
>
>
>     On Mon, Oct 4, 2021 at 10:18 AM Joel Serrano <joel at textplus.com
>     <mailto:joel at textplus.com>> wrote:
>
>         Hey guys, 
>
>         I'm trying to do a replacement regex using subst_hf but I
>         can't seem to get the format right and the docs are confusing.
>
>         *subst_hf: *docs say it uses perl-like regex (as I understand
>         it, that would mean PCRE). Docs also say in the 'subexp' param
>         that it uses the same format as *subst.*
>         *
>         *
>         *subst: *at the beginning it says it uses perl or sed format,
>         but in the '/re/repl/flags' param it says that sed format.
>
>         https://www.kamailio.org/docs/modules/devel/modules/textops.html#textops.f.subst_hf
>         <https://www.kamailio.org/docs/modules/devel/modules/textops.html#textops.f.subst_hf>
>
>
>               4.20.  |subst_hf(hf, subexp, flags)|
>
>         Perl-like substitutionsin the body of a header field.
>
>         Meaning of the parameters is as follows:
>
>          *
>
>             /hf/ - header field name.
>
>          *
>
>             /subexp/ - substitution expression in the same format as
>             of the 'subst' functionparameter.
>
>          *
>
>             /flags/ - control flags - it has to be one of: a - all
>             headers matching the name; f - only first header matching
>             the name; l - only the last header matching the name.
>
>
>               4.16.  |subst('/re/repl/flags')|
>
>         Replaces re with repl (sed or perl like).
>
>         Meaning of the parameters is as follows:
>
>          *
>
>             /'/re/repl/flags'/ - sed like regular expression. flags
>             can be a combination of i (case insensitive), g (global)
>             or s (match newline don't treat it as end of line).
>
>             're' - is regular expression
>
>             'repl' - is replacement string - may contain pseudo-variables
>
>             'flags' - substitution flags (i - ignore case, g - global)
>
>
>
>
>
>         I'm trying to use this:
>
>         subst_hf("X-myheader", "/^(?!string1 |example2-|text3
>         )(.*)/myprefix \1/", "a")
>
>         In an online tester using PCRE, the format is correct, but
>         Kamailio doesn't like it:
>
>         Oct  4 09:29:16 cops-n1 cops[15957]: ERROR: <core>
>         [core/re.c:285]: subst_parser(): bad regular expression
>         ^(?!string1 |example2-|text3 )(.*) in
>         /^(?!string1 |example2-|text3 )(.*)/myprefix \1/
>
>
>         If I remove the negative lookahead chars (?!) then kamailio
>         doesn't complain but the regex doesn't do what I want.
>
>         Does this mean that Kamailio won't support negative lookahead
>         for subst regex replacements?
>
>         What I'm trying to achieve is something along:
>
>         "If the value of the header Myheader doesn't start with ABC,
>         DEF or HIJ, then prefix it with XYZ"
>
>         Any suggestions/ideas on how to achieve this?
>
>
>         Thanks, 
>         Joel.
>
>
>
>
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-- 
Daniel-Constantin Mierla -- www.asipto.com
www.twitter.com/miconda -- www.linkedin.com/in/miconda
Kamailio Advanced Training - Online
Nov 08-11, 2021 (Europe Timezone) - Nov 22-25, 2021 (America Timezone)
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