At the third time there are no further matches (dry pass as you call it) and so that is a relatively cheap pass. I found only a small performance difference between one or the other (<8%). Another consideration is that using the loop means short code that is easy to understand.

---------- Post updated at 21:22 ---------- Previous update was at 21:05 ----------

We could simplify the two statements since we can do the first field in the first run and the last field in the second run:

Code:

sed -r 's/(,|^)  *,/\1,/g;s/,  *(,|$)/,\1/g'

This runs 20% faster.

---------- Post updated at 21:30 ---------- Previous update was at 21:22 ----------

OK, this really is lots and lots faster; it only takes 1/5th of the time !!

Code:

sed -r 's/^  *,/,/;s/,  *,/,,/g;s/,  *,/,,/g;s/,  *$/,/'

but it is not because of the absence of the loop, since:

Code:

sed -r 's/^  *,/,/;:a;s/,  *,/,,/g;ta;s/,  *$/,/'

this is only 10% slower than this fast solution.

Obviously we can now drop the -r:

Code:

sed 's/^  *,/,/;s/,  *,/,,/g;s/,  *,/,,/g;s/,  *$/,/'

Code:

sed 's/^  *,/,/;:a;s/,  *,/,,/g;ta;s/,  *$/,/'

But that made no difference in performance.

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Apparently it is the lack of capturing groups, alternation and back references that makes all the difference!!
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