The WordStart command returns the index of the beginning (first letter) of the word containing the index index, if any. If the second argument index does not occur in a word in the string s, then the value  is returned.

The WordEnd command returns the index of the end (last letter) of the word containing the index index, if any. If the second argument index does not occur in a word in the string s, then the value  is returned.

The WordContaining command returns the word containing the index index, if index lies within a word in the string s, or the empty string , otherwise.

For all three commands, the second argument index must be a valid index into the string presented as the first argument.

If the index index lies within a word in the string s, then WordContaining( s, index ) = s[ WordStart( s, index ) .. WordEnd( s, index ) ].