import re

def filename_matches_pattern(filename, pattern):
    return bool(re.fullmatch(pattern, filename))

# Test
filename = "example.txt"
pattern = r'^[a-zA-Z0-9_]+\.(txt|csv)$'  # Pattern to match alphanumeric filename with .txt or .csv extension
print(filename_matches_pattern(filename, pattern))  # True or False