There is either a bug in perl, or in my understanding of the /n regular-expression option. (This is perl v5.24.3, in case behavior varies among releases.) 
#!/usr/bin/perl

$_ = 'abcdefg';        # define a string

/(..)(..)(..)/;        # break the string into 3 two-character chunks

print "$1 $2 $3\n";    # print the chunks

/(j)/n;                # search for a letter that's not in the string

print "$1 $2 $3\n";    # print the chunks

/(g)/n;                # search for a letter that's in the string

print "$1 $2 $3\n";    # print the chunks

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Because the latter two regular expressions use the /n option, they should have no effect on the existing values of $1, $2, and $3. And in fact the first one does not. But the second one erases all three.

I grant you, the documentation for /n is a little sketchy, saying only "Non-capture mode. Don't let () fill in $1, $2, etc..." It doesn't actually specify what happens to existing values of these variables. But I would expect consistent behavior: it should either always preserve the values (and this seems the ideal — what is ever gained from overwriting them when the user has asked that they not be populated?), or always erase them. Erasing them in the case of a successful match but not a failed one is at least unexpected, if it doesn't rise to the level of outright bug.

Or, I'm fundamentally misunderstanding something about /n. What says the wisdom of the monastery?

In reply to non-capture mode sometimes erases previous capture by raygun

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