import re
 
data = """***   DATA
     1    253.31     78.20     490.0         0 0 1 0 0
   101         0         0         0         0         0         0          
     1         2         3         4         5         6
     .
     .
     .
     .
    10         A         B         C         0 0 1 0 0
   110         0         0         0         0         0         0          
    20        21        22        23        24        25"""
mldata = data.split('\n')
 
regex = re.compile(r'\b([A-Za-z])\b')
replacement = "test"
 
for line in mldata:
	newline = re.sub(regex,replacement,line)
	print newline
 

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