I made a litle change youre regexp and tried to analyze 

my @data = (-123.004,-.008,0,-0,.0987,1.1,12345,'d','test');


foreach my $value (@data){
        if ( $value =~ /(^-?)(\d?|\d+)(\.?)(\d?|\d+)$/){
                print "$1 - $2 -  $3 - $4  true\n";
        }
        else{
                print "false\n";
        }
}

[download]

For this analyze i ignored the process of $1 - first grouping, i think i can understand that, so we straight to second grouping

For the $value = 123.004
1. Move on to second group and pick the first alternative (\d?)
2. "123" doesnt match (\d?), since (\d?) = match digit 1 or 0 times
3. Backtrack 1 character.
4. Pick the second alternative in the second group (\d+) or match digit 1 or more times, and it match, so we got "123" for $2
5. Move on to the third group, (\.?) match "." 1 or 0 times, so we got "." for $3
6. Move on to the fourth group, first alternative doesnt match so backtrack 1 character, try the second alternative and match "004", so we got "004" for $4.

for the $value = 12345
1. Move on to the second group and pick the first alternative (\d?)
2. "12345" doesnt match (\d?)
3. backtrack 1 character
4. Pick the second alternative in the second group (\d+) or match digit 1 or more times, and we got "1" for $2, i am not sure about this, i keep thinking that we should get "12345" for $2 (is there something in third grouping (\.?) ?)
5. Move on to the third group, "2345" doesnt match (\.?)
6. Move on to the fourth group, "2345" doesnt match (\d?), (\d?) only match "2" in "2345".
7. backtrack 1 character, try the second alternative (\d+) and "2345" match (\d+) or match digit 1 or more times.

sorry for my english, zak

In reply to Re^2: combined into a single regex by doctor_moron
in thread combined into a single regex by arcnon

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