^start of line
\s*any whitespace character, repeated any number of times
Non-Capturing Group(?:\S+\s+){0,3} {0,3}repeats the group contents below between 0 and 3 times:
\S+any non-whitespace character, repeated at least once
\s+any whitespace character, repeated at least once
Non-Capturing Group(?:cos|because) costhe literal text cos (case sensitive) becausethe literal text because (case sensitive) \ban ASCII word boundary
.*any character (except for line terminators), repeated any number of times
(*SKIP)acts like (*PRUNE), but the next unanchored match attempt starts where (*SKIP) was reached instead of at the next subject character
(*F)forces the current match attempt to fail at this position