? Do you mean a space after the slash? In your quotes it looks like there's a space after.
Is the "a" in the middle of a word or on its own?
It's almost always best to post your xml so we can see the context. Maybe the following will help but it's hard to tell without knowing the context.
<?xml version="1.0" encoding="utf-16"?>
<!--VoxCommando 1.9.1.9-->
<command id="270" name="a to the b" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="Results.Replace (no regular expression) is sometimes enough, and sometimes easier. Sometimes not.

The 2nd, "replace with", parameter in Results.RegExReplace does not take regular expressions. So if you want spaces around your b, put spaces around your b.">
<action>
<cmdType>Results.SetLastResult</cmdType>
<params>
<param>I am a on my own.</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>OSD.ShowText</cmdType>
<params>
<param>{LastResult}</param>
<param>10000</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>Results.Replace</cmdType>
<params>
<param> a </param>
<param> b </param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>OSD.AddText</cmdType>
<params>
<param>{LastResult}</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>Results.SetLastResult</cmdType>
<params>
<param>I am ab when I should be just b. But fab is still fab.</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>OSD.AddText</cmdType>
<params>
<param>{LastResult}</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>Results.RegExReplace</cmdType>
<params>
<param>(\sab\s)</param>
<param> b </param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>OSD.AddText</cmdType>
<params>
<param>Now, {LastResult}</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>Results.SetLastResult</cmdType>
<params>
<param>Sometimes "results.replace" is/a/better choice than "results.regex".</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>OSD.AddText</cmdType>
<params>
<param>{LastResult}</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>Results.Replace</cmdType>
<params>
<param>/</param>
<param> / </param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>OSD.AddText</cmdType>
<params>
<param>{LastResult}</param>
</params>
<cmdRepeat>1</cmdRepeat>
</action>
</command>