Allow two word selection behaviours at the same time in Sublime.

Default (Linux).sublime-mousemap

[
  {
    "button": "button1",
    "modifiers": ["alt"],
    "command": "select_underscored_word",
  },
]

select_underscored_word_plugin.py

import re

import sublime
import sublime_plugin


class SelectUnderscoredWordCommand(sublime_plugin.TextCommand):
    """Select whole word udner cursor, even if containing underscores.

    That is actually Sublime's default, but is e.g. overriden when
    adding a '_' to the word_separators setting. Best illustrated with
    an example:

        Selecting word under cursor (the '|'):

            '_my_variab|le_name'
                'variab|le'

        With this command it would be:

            '_my_variab|le_name'
            '_my_variab|le_name'

    So this can be used to make use of both selection behaviours in
    parallel. With the keyboard, you may use the 'select single word
    without underscore' behaviour, while e.g. an Alt-mouseclick 
    (preferred to a doubleclick as that would overwrite things like 
    jump-to-next-search-result) could bind to this command to trigger 
    the 'select entire word including underscores' behaviour.

    /Packages/User/Default (Linux).sublime-mousemap

    [
      {
        "button": "button1",
        "modifiers": ["alt"],
        "command": "select_underscored_word",
      },
    ]

    """

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

        cursors = view.sel()
        if len(cursors) != 1:
            return

        region = cursors[0]
        line = view.line(region)

        region_begin, region_end = region.begin(), region.end()
        if region_begin == region_end:
            # Nothing selected, let's assume we selected the next char.
            region_end = region_begin + 1
        fixed_region = sublime.Region(region_begin, region_end)

        start = fixed_region.begin() - line.begin()
        end = start + fixed_region.end() - fixed_region.begin() - 1

        line_text = view.substr(line)
        before = line_text[:start]
        after = line_text[end:]

        pattern = r"[\w]"

        select_next_n_chars = 0
        for char in after:
            if not re.match(pattern, char):
                break
            select_next_n_chars += 1

        select_prev_n_chars = 0
        for char in reversed(before):
            if not re.match(pattern, char):
                break
            select_prev_n_chars += 1

        new_region = sublime.Region(
            fixed_region.begin() - select_prev_n_chars,
            fixed_region.end() + select_next_n_chars - 1,
        )

        self.view.sel().clear()
        self.view.sel().add(new_region)