[SR-Users] Understanding regex format for subst_hf

Joel Serrano joel at textplus.com
Mon Oct 4 19:46:22 CEST 2021


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> 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
>
> 4.20.  subst_hf(hf, subexp, flags)
>
> Perl-like substitutions in 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' function parameter.
>    -
>
>    *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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