Regression for text with leading underscores

[justanotheranonymoususer]

Version: 8c74601

import markdown2

# Expected output (for all three cases):
# <p><strong>__a</strong> b <strong>__c</strong> d <em>e</em></p>
# Actual output:
# <p>**<strong>a** b **</strong>c** d</p>
# <p>__a** b <strong>__c</strong> d</p>
# <p>**<strong>a** b **</strong>c** d</p>
markdown_text = R"""
**__a** b **__c** d *e*
"""
print(markdown2.markdown(markdown_text))
print(markdown2.markdown(markdown_text, extras={'code-friendly': None}))
print(markdown2.markdown(markdown_text, extras={'middle-word-em': {'allowed': False}}))