I found a solution that works, but to gain more understanding (and I'm stubborn that way) I want to know why my first solution does not work.

I'm parsing a file, looking for a single line to replace 2 characters.
Line in question:
__msr2f6:       data8 0b00000000000000000000000000_1_0_000000000000001
+0101_00000000000000000

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My initial search/replace was this: 
$line =~ s/^(__msr2f6:\s+data8 0b[01]+_)[01]_[01](_[01]+_[01]+)/$10_0$
+2/;

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This causes an error to be printed: 
Use of uninitialized value in concatenation (.) at myscript.pl line 33
+, <FILE> line 915.

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A co-worker suggested a change, but didn't know why it might work:
$line =~ s/^(__msr2f6:\s+data8 0b[01]+)_[01]_[01]_([01]+_[01]+)/$1_0_0
+_$2/;

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So I moved the leading and trailing underscore out of the parens and put them in the replace string.

Here are some things I think I understand, but then again maybe not...

1. Backrefrences are 1-9, only a single digit.
2. Underscores are valid variable name components.

So what changes between these two examples? If the first variable is being treated as $10_0, then why is the second not being treated as $1_0_0_?

This solution also works:

$line =~ s/^(__msr2f6:\s+data8 0b[01]+_)[01]_[01](_[01]+_[01]+)/$1."0_
+0".$2/e;

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But it is probably slower than the other working solution.

Thanks for the enlightenment!

In reply to search, replace and backrefrences by Helter

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