https://chromium.googlesource.com/apps/libapps/+/master/hterm/js/hterm_screen.js#850

hterm.Screen.prototype.expandSelection has a few lines where it hardcodes the word break characters:
  // Matches can start with '~' or '.', since paths frequently do.
  var leftMatch   = '[^\\s\\[\\](){}<>"\'\\^!@#$%&*,;:`]';
  var rightMatch  = '[^\\s\\[\\](){}<>"\'\\^!@#$%&*,;:~.`]';
  var insideMatch = '[^\\s\\[\\](){}<>"\'\\^]*';

we should probably pull those out into the options for users to customize.