Variables with single character are not processed

[CammilleCC]

Received a report from Loic that variables with single character are not being processed. From his screenshot of the trace, it seems that the variable name regex check fails here. I've successfully replicated the bug:

>>> import re
>>> re.fullmatch(r"[a-zA-Z_]\w+", 'x', flags=re.A)
>>> re.fullmatch(r"[a-zA-Z_]\w+", 'X', flags=re.A)
>>> re.fullmatch(r"[a-zA-Z_]\w+", 'x_pos', flags=re.A)
<re.Match object; span=(0, 5), match='x_pos'>
>>> re.fullmatch(r"[a-zA-Z_]\w+", 'X_pos', flags=re.A)
<re.Match object; span=(0, 5), match='X_pos'>