I was surprised that you could not simply write
.rst stands for **r**e**s**tructured **t**ext.
but the reStructuredText specification indeed states that inline markup must be followed by white-space or one of - . , : ; ! ? \ / ' " ) ] } or >, so the above string of reStructuredText is not valid. However, only a minor change is required to get valid character markup with backslash escapes. Changing the above to
.rst stands for **r**\ e\ **s**\ tructured **t**\ ext.
works fine. To see this in action try the online reST to HTML converter.