What characters are allowed in tensorflow variable names?

When choosing a name for a tensorflow variable, this is ok:

>> tf.Variable(2, name='a')

<tf.Variable 'finegrained_1/decoder/unreadable/a:0' shape=() dtype=int32_ref>

However, this is not:

>> tf.Variable(2, name='a:b')

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
~/mm/appy-finegrained/sign_classification/model_finegrained.py in <module>()
----> 1 tf.Variable(2, name='a:b')

~/anaconda3/lib/python3.6/site-packages/tensorflow/python/ops/variables.py in __init__(self, initial_value, trainable, collections, validate_shape, caching_device, name, variable_def, dtype, expected_shape, import_scope, constraint)
    211           dtype=dtype,
    212           expected_shape=expected_shape,
--> 213           constraint=constraint)
    214 
    215   def __repr__(self):

~/anaconda3/lib/python3.6/site-packages/tensorflow/python/ops/variables.py in _init_from_args(self, initial_value, trainable, collections, validate_shape, caching_device, name, dtype, expected_shape, constraint)
    287     with ops.control_dependencies(None):
    288       with ops.name_scope(name, "Variable", [] if init_from_fn else
--> 289                           [initial_value]) as name:
    290 
    291         if init_from_fn:

~/anaconda3/lib/python3.6/site-packages/tensorflow/python/framework/ops.py in __enter__(self)
   4930       self._g_manager.__enter__()
   4931       self._name_scope = g.name_scope(self._name)
-> 4932       return self._name_scope.__enter__()
   4933 
   4934   def __exit__(self, type_arg, value_arg, traceback_arg):

~/anaconda3/lib/python3.6/contextlib.py in __enter__(self)
     79     def __enter__(self):
     80         try:
---> 81             return next(self.gen)
     82         except StopIteration:
     83             raise RuntimeError("generator didn't yield") from None

~/anaconda3/lib/python3.6/site-packages/tensorflow/python/framework/ops.py in name_scope(self, name)
   3512         # (viz. '-', '\', '/', and '_').
   3513         if not _VALID_SCOPE_NAME_REGEX.match(name):
-> 3514           raise ValueError("'%s' is not a valid scope name" % name)
   3515       else:
   3516         # Scopes created in the root must match the more restrictive

ValueError: 'a:b' is not a valid scope name

Putting a / in a name is allowed, but it probably breaks some scoping things:

>> tf.Variable(2, name='a/b')

<tf.Variable 'finegrained_1/decoder/unreadable/a/b:0' shape=() dtype=int32_ref>

Are there any defined rules as to what character set is allowed in variable names?

And (with regards to the a/b name) are there any extra guidelines for what shouldn't be used?

Solution

As for ops naming, see tf.Operation documentation:

NOTE: This constructor validates the name of the Operation (passed as node_def.name). Valid Operation names match the following regular expression:
[A-Za-z0-9.][A-Za-z0-9_.\\-/]*

Each variable is translated into a number of ops, and its name is put into the scope, which naming rules are very similar (see python/framework/ops.py):

_VALID_OP_NAME_REGEX = re.compile("^[A-Za-z0-9.][A-Za-z0-9_.\\-/]*$")
_VALID_SCOPE_NAME_REGEX = re.compile("^[A-Za-z0-9_.\\-/]*$")

Naturally, '/' and ':' are not allowed in the name, since they are used for internal tensorflow purposes:

'/' is a separator of nested scopes (the regex allows it, but this means using several scopes in a single declaration);
':' is a separator of tensor name and output index (see this question for details).