[sr-dev] [kamailio/kamailio] core: allow empty matches (#2869)

[Bastian Triller]

Anchors like ^ and $ match beginning resp. ending of strings but their
matches are zero length, which is ok.
The anchors can be used to pre-/append a prefix/suffix via e.g.
subst_hf("X-My-Header", "/^/prefix /", "a");
subst_hf("X-My-Header", "/$/ suffix/", "a");

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#### Type Of Change
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#### Checklist:
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#### Description
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You can view, comment on, or merge this pull request online at:

  https://github.com/kamailio/kamailio/pull/2869

-- Commit Summary --

  * <a href="https://github.com/kamailio/kamailio/pull/2869/commits/63190fb0f0580a276146552f14d2ee745e6d3110">core: allow empty matches</a>

-- File Changes --

    M src/core/data_lump.c (2)
    M src/core/re.c (4)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/2869.patch
https://github.com/kamailio/kamailio/pull/2869.diff

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