hexdump = """. a . v a l i d . s t r i n g . 
. a n o t h e r . s t r i n g . 
. e t c . . . . . . . . . . . . 
l i n e . w . a .   s p a c e . 
. e t c . . . . . . . . . . . . 
"""
from pyparsing import oneOf, printables, delimitedList, White, LineEnd
dumpchar = oneOf(list(printables) + [' ']).leaveWhitespace()
dumpchar.setParseAction(lambda t: '_' if t[0] == '.' else None)
dumpline = delimitedList(dumpchar, delim=White(' ', exact=1)) + LineEnd(
    ).suppress()
for t in dumpline.searchString(hexdump):
    pass
Pyparsing: space as a valid token