To match $a only in data not enclosed in tags you need a regex of 
/>[^<]*$a/s;

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to replace $a with $b only in data not enclosed in tags you need 
s/(>[^<]*)$a/$1$b/sg;

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In reply to Re: Replace characters not in html tags by dingus
in thread Replace characters not in html tags by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
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