%letterBoundary Creates a pattern that matches the beginning or end of a substring of letters
%   PAT = letterBoundary returns a pattern that matches the beginning or end of
%   a substring of letters.
%
%   PAT = letterBoundary(TYPE) returns a pattern that matches the TYPE of
%   letter boundary. TYPE can be 'start', 'end', or 'either'.
%
%   Examples
%       STR = "Four score" + newline + "and seven years ago";
%
%       PAT = letterBoundary + "a" + lettersPattern;
%       extract(STR,PAT)                   % returns  ["and";"ago"]
%
%       PAT = lettersPattern + "o" + letterBoundary;
%       extract(STR,PAT)                   % returns  "ago"
%
%   See also pattern, textBoundary, digitBoundary, contains, extract, newline.

%   Copyright 2019-2020 The MathWorks, Inc.