> But I can't understand why it doesn't work if you put the \s* outside the parens, nor I can understand why it stops working if I put the \s*$ at the end of the regex :-(
That is because if you have the string
root: email@example.com 11111233333333333333333333
and the RE
then the part A in the RE matches the beginning of the string, part BBBBB matches 11111 ("root:") and CCC matches an empty string (not a space, a string with zero chars in it). After this empty string follows a space, and the space is not the beginning of "firstname.lastname@example.org", because it is the beginning of " email@example.com".
I hope things are getting clearer for you :-)