BUG: pow operator without whitespace errors

[jonas-eschle]

Hi all, thanks a lot for the great package!

There seems to be a bug with missing whitespaces and the power operator **. It can only be parsed if a whitespace is inserted before.

formulate.from_numexpr('a**2') raises an error
formulate.from_numexpr('a ** 2') works

Other operators work fine without whitespace.

This is the full error:

In [12]: formulate.from_numexpr('a**2')                                         
ERROR:formulate:TODO TRACEBACK: ('a**2', 1, 'Expected end of text')
ERROR:formulate:Error parsing: a**2
ERROR:formulate:                ▲
ERROR:formulate:                ┃
ERROR:formulate:                ┗━━━━━━ Error here or shortly after
---------------------------------------------------------------------------
ParseException                            Traceback (most recent call last)
~/anaconda3/envs/rkq37/lib/python3.7/site-packages/formulate/parser.py in to_expression(self, string)
    232         try:
--> 233             result = self._parser.parseString(string, parseAll=True)
    234             assert len(result) == 1, result

~/anaconda3/envs/rkq37/lib/python3.7/site-packages/pyparsing.py in parseString(self, instring, parseAll)
   1954                     exc.__traceback__ = self._trim_traceback(exc.__traceback__)
-> 1955                 raise exc
   1956         else:

~/anaconda3/envs/rkq37/lib/python3.7/site-packages/pyparsing.py in parseImpl(self, instring, loc, doActions)
   3813         if loc < len(instring):
-> 3814             raise ParseException(instring, loc, self.errmsg, self)
   3815         elif loc == len(instring):

ParseException: Expected end of text, found '*'  (at char 1), (line:1, col:2)

During handling of the above exception, another exception occurred:

ParsingException                          Traceback (most recent call last)
<ipython-input-12-c3a50f695328> in <module>
----> 1 formulate.from_numexpr('a**2')

~/anaconda3/envs/rkq37/lib/python3.7/site-packages/formulate/parser.py in to_expression(self, string)
    244             exception = ParsingException()
    245             exception.__context__ = None
--> 246             raise exception
    247         else:
    248             return result

ParsingException: 

Any ideas?

[eduardo-rodrigues]

Allow me to ping @chrisburr as this bug is probably an easy fix for an expert.