Note: Perl regexp matching is not necessarily implemented as described below. I'm totally ignorant as to how it is actually implemented. One could say this document describes the specs rather than the implementation.
It has nothing to do with lookaheads, really. For example, let's look at
/^ab*bc/
The regexp can be read as:
1. Starting at the begining of the string
2. Match an 'a'.
3. Match as many 'b's as possible, but not matching any is ok.
4. Match a 'b'.
5. Match a 'c'.
Match against 'abbbbbbc'
01234567
1) ok! pos = 0. (zw)
2) ok! Found an 'a' at pos 0. pos = 1.
3) ok! Found 6 'b's at pos 1 through 6. pos = 7.
4) fail! Did not find a 'b' at pos 7. Backtrack!
3) ok! Found 5 'b's at pos 1 through 5. pos = 6.
4) ok! Found a 'b' at pos 6. pos = 7.
5) ok! Found a 'c' at pos 7. pos = 8.
Match!
Something similiar is occuring with your
/^root:\s*(?!email)/
The regexp can be read as:
1. Starting at the begining of the string
2. Match 'root:'.
3. Match as many '\s's as possible, but not matching any is ok.
4. Match something other than 'email'.
Match against 'root: email'
01234567890
1) ok! pos = 0. (zw)
2) ok! Found a 'root:' at pos 0 through 4. pos = 5.
3) ok! Found 1 '\s' at pos 5. pos = 6.
4) fail! Found 'email' at pos 6 through 10. Backtrack!
3) ok! Found 0 '\s' at pos 5. pos = 5. (zw)
4) ok! Found something other than 'email' at pos 5. pos = 5. (zwla)
(found ' email')
Match!
Now let's look at my solution
/^root:\s*(?!email)\S/
The regexp can be read as:
1. Starting at the begining of the string
2. Match 'root:'.
3. Match as many '\s's as possible, but not matching any is ok.
4. Match something other than 'email'.
5. Match a '\S'.
Match against 'root: email'
01234567890
1) ok! pos = 0. (zw)
2) ok! Found a 'root:' at pos 0 through 4. pos = 5.
3) ok! Found 1 '\s' at pos 5. pos = 6.
4) fail! Found 'email' at pos 6 through 10. Backtrack!
3) ok! Found 0 '\s' at pos 5. pos = 5. (zw)
4) ok! Found something other than 'email' at pos 5. pos = 5. (zwla)
(found ' email')
5) fail! Did not find a '\S' at pos 5. Backtrack!
Nothing more to try.
No match!
Match against 'root: hisemail'
01234567890123
1) ok! pos = 0. (zw)
2) ok! Found a 'root:' at pos 0 through 4. pos = 5.
3) ok! Found 1 '\s' at pos 5. pos = 6.
4) ok! Found something other than 'email' at pos 6. pos = 6. (zwla)
(found 'hisemail')
5) ok! Found a '\S' at pos 6. pos = 6.
Match!
Backtracking means: (might not be an exhaustive list)
 In the case of the first rule
 Look for a match further on.
 In the case of a * rule or ? rule,
 try matching less.
 In the case of a *? rule or ?? rule,
 try matching more.
 In the case of a  or [] rule,
 try matching the next choice.
 else,
 no match, so backtrack the last matching rule.
