An alternative to Sublime Text 2's built-in join_lines command, which contains an annoying ideosyncracy of reaching outside the selected region when performing a join.

join_lines_alt.py

# -*- coding: UTF-8 -*-

import sublime_plugin


# An alternative to the builtin "join_lines" command,
# which contains an idiosyncracy that I consider a bug.

class JoinLinesAltCommand(sublime_plugin.TextCommand):

    def run(self, edit):
        view = self.view

        # First, join the lines in each selection together.
        # (Taking care not to join empty lines.)
        #
        # Using view.sel()[] directly because it gets updated as we make replacements,
        # whereas the region objects returned by it do not.
        join_regex = r"(?<=\S)[ \t]*\n[ \t]*(?=\S)"
        for c in xrange(len(view.sel())):
            # If the selection is just a caret,
            # assume the user wants to join the following line.
            if len(view.sel()[c]) == 0:
                line = view.full_line(view.sel()[c].begin())
                m = view.find(join_regex, line.begin())
                if line.intersects(m):
                    view.replace(edit, m, " ")

            # If the selection contains something,
            # only perform joins INSIDE the selected region
            else:
                m = view.find(join_regex, view.line(view.sel()[c].begin()).begin())
                while m and view.sel()[c].intersects(m):
                    view.replace(edit, m, " ")
                    m = view.find(join_regex, m.begin())

    def is_enabled(self):
        return self.view.sel()