https://bugs.exim.org/show_bug.cgi?id=2645
Philip Hazel <Philip.Hazel@???> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |INVALID
Status|NEW |RESOLVED
--- Comment #1 from Philip Hazel <Philip.Hazel@???> ---
This is not a bug. It is a documented effect. There is an option to change the
default behaviour; this is what the documentation says:
"PCRE2_SUBSTITUTE_UNSET_EMPTY causes unset capture groups (including unknown
groups when PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set) to be treated as empty
strings when inserted as described above. If this option is not set, an attempt
to insert an unset group causes the PCRE2_ERROR_UNSET error. This option does
not influence the extended substitution syntax described below."
The extended syntax (applied when the PCRE2_SUBSTITUTE_EXTENDED option is set)
provides a more extensive facility:
${<n>:-<string>}
${<n>:+<string1>:<string2>}
"As before, <n> may be a group number or a name. The first form specifies a
default value. If group <n> is set, its value is inserted; if not, <string> is
expanded and the result inserted. The second form specifies strings that are
expanded and inserted when group <n> is set or unset, respectively."
If your regex101 example is using PCRE2, presumably it is setting the
PCRE2_SUBSTITUTE_UNSET_EMPTY option.
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