Regular expression in BBEdit: find and replace within a context

I came across a generated HTML file with reference to thousands of .aif audio files that had spaces in their names. I had no problem correcting the file names themselves, using a perl script: replacing " " to "_". But updating the links within the html file was a problem, I have very limited knowledge of regular expression, and the little that I know, I have found it very useful. My problem in this case is I need to replace " " to "_" with a link, not globally:

I first tried this:

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Find this:

(a href="[^ ]+) <--note a space after the ending)

Replace with:

\1_

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Very crude...it would work to a point but then it started replacing spaces that are in non link text for example, "<a href="abc.aif">the_link..."which I didn't want to happen.

It took a while before I managed to come up with something like this:

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Find this:

(^<a href="[^ ]+) ([^ ]+\.aif">$)

Replace with:

\1_\2

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This would of course only look for the first "_" within the link. I had to run it multiple times in order to change names like "abc def.aif" to "abc___def.aif". I also came across some names that are like "ab cd ef.aif". I changed my grep patten in order to accommodate this.

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Find this:

(<a href="[^ ]+) ([^ ]*) ([^ ]+\.aif">) <--note the 2nd match

Replace with:

\1_\2_\3

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I will probably come back here to check how i did it. If you have a better solution, please let me know.