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DOCS-958: Clarification on $regex use of indexes #525

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Jan 8, 2013
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DOCS-958: Clarification on $regex use of indexes #525

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15 changes: 8 additions & 7 deletions source/reference/operator/regex.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -73,10 +73,11 @@ $regex
``collection`` that match the case insensitive regular expression
``acme.*corp`` that *don't* match ``acmeblahcorp``.

:operator:`$regex` uses :term:`indexes <index>` only when the
regular expression has an anchor for the beginning (i.e. ``^``)
of a string. Additionally, while ``/^a/``, ``/^a.*/``, and
``/^a.*$/`` are equivalent, they have different performance
characteristics. All of these expressions use an index if an
appropriate index exists; however, ``/^a.*/``, and ``/^a.*$/``
are slower. ``/^a/`` can stop scanning after matching the prefix.
:operator:`$regex` can only use an :term:`index <index>` efficiently
when the regular expression has an anchor for the beginning (i.e. ``^``)
of a string and is a case-sensitive match. Additionally, while
``/^a/``, ``/^a.*/``, and ``/^a.*$/`` match equivalent strings, they
have different performance characteristics. All of these expressions
use an index if an appropriate index exists; however, ``/^a.*/``,
and ``/^a.*$/`` are slower. ``/^a/`` can stop scanning after matching
the prefix.