caption_hashtags property for only-if evaluation

caption_hashtags is a list of all hashtags that are mentioned in the
Post's caption. It allows to easily filter Posts that have multiple
hashtags, and as such fixes #24.

Further, the documentation of --only-if has been completed by linking to
a description of the syntax in the Python documentation, and by linking
to a list of all defined properties with their meanings. So, this commit
also closes #42.

README.rst

@@ -129,8 +129,8 @@ Filter Posts
 The ``--only-if`` option allows to specify criterias that posts have to
 meet to be downloaded. If not given, all posts are downloaded. It must
 be a boolean Python expression where the variables ``likes``,
-``comments``, ``viewer_has_liked``, ``is_video``, ``date``, and some
-more (see class ``instaloader.Post`` for a full list) are defined.
+``comments``, ``viewer_has_liked``, ``is_video``, and many
+more are defined.
 
 A few examples:
 
@@ -153,8 +153,17 @@ Or you may **skip videos**:
 
     instaloader --only-if="not is_video" target
 
+Or you may filter by hashtags that occur in the Post's caption. For
+example, to download posts of kittens that are cute: ::
+
+    instaloader --only-if="'cute' in caption_hashtags" "#kitten"
+
 .. basic-usage-end
 
+(For a more complete description of the ``-only-if`` option, refer to
+the `Instaloader Documentation <https://instaloader.readthedocs.io/basic-usage.html#filter-posts>`__)
+
+
 Advanced Options
 ----------------
 

docs/as-module.rst

@@ -16,6 +16,8 @@ Introduction
    :members:
    :undoc-members:
 
+.. _post-class:
+
 ``Post`` Class
 ^^^^^^^^^^^^^^
 

docs/basic-usage.rst

@@ -7,3 +7,9 @@ Basic Usage
 .. include:: ../README.rst
    :start-after: basic-usage-start
    :end-before: basic-usage-end
+
+.. (continuation of --only-if explanation)
+
+The given string is evaluated as a
+`Python boolean expression <https://docs.python.org/3/reference/expressions.html#boolean-operations>`__,
+where all occuring variables are properties of the :ref:`post-class`.

instaloader.py

@@ -275,6 +275,16 @@ class Post:
         elif "caption" in self._node:
             return self._node["caption"]
 
+    @property
+    def caption_hashtags(self) -> List[str]:
+        """List of all hashtags (without preceeding #) which occur in the Post's caption."""
+        if not self.caption:
+            return []
+        # This regular expression is from jStassen, adjusted to use Python's \w to support Unicode
+        # http://blog.jstassen.com/2016/03/code-regex-for-instagram-username-and-hashtags/
+        hashtag_regex = re.compile(r"(?:#)(\w(?:(?:\w|(?:\.(?!\.))){0,28}(?:\w))?)")
+        return re.findall(hashtag_regex, self.caption)
+
     @property
     def is_video(self) -> bool:
         """True if the Post is a video."""