Hi Joaquin,
My guess is that keywords are whitespace separated character runs – they don't contain whitespace.
BBEdit probably chunks the keywords "before" matching against the regular expression chunk by chunk.
For example what your regular expression matches against is the string "label2:", not the whole line. Thus the start of line anchor always applies.
The Numeric Tags appear correctly because they are excluded from the "Number Pattern" not because of the start of line anchor.
I fear that keyword patterns won't let you do what you want. You will have to use the other patterns that are whitespace aware… but there is a limited number of those.
HTH,
Jean Jourdain