Extracts fields (by position or span) split by whitespace or chained delimiter rules
StringExtract["string", n]
StringExtract["string", {pos1, pos2, …}]
StringExtract["string", sep -> pos]
StringExtract["string", pos1, pos2, …]
StringExtract["string", sep1 -> pos1, sep2 -> pos2, …]
StringExtract["a bbb cccc aa d", 2]
→ bbbStringExtract["a bbb cccc aa d", 2 ;; 4]
→ {bbb, cccc, aa}StringExtract["a--bbb--ccc--dddd", "--" -> 3]
→ ccc