Incorrect highlighting when || is not followed by whitespace

[marshallward]

(Re-posting on behalf of @philj56 ; original text below)

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The following example shows the issue:

.. math::
|E| + 3
|E|+3

The first line highlights correctly, with the + 3 using normal maths highlighting. On the second line, the lack of whitespace causes the |E| highlighting to never end, and carries on through the whole document until another || pair followed by whitespace is found.

Edit: Actually this doesn't just apply to math mode, the same thing happens without the directive.