The second test’s (.*) groups match and retain any number of characters. The third
and fourth tests shows how alternatives can be grouped by both position and name,
and the last test matches C #define lines—more on this pattern in a moment:
C:\...\PP4E\Lang> python re-groups.py
0 1 2
('000', '111', '222')
('A', 'Y', 'C')
{'a': 'A', 'c': 'C', 'b': 'Y'}
('spam', '1 + 2 + 3')Finally, besides matches and substring extraction, re also includes tools for string re-
placement or substitution (see Example 19-5).
Example 19-5. PP4E\Lang\re-subst.py
"substitutions: replace occurrences of pattern in string"
import re
print(re.sub('[ABC]', '', 'XAXAXBXBXCXC'))
print(re.sub('[ABC]_', '', 'XA-XA_XB-XBXC-XC')) # alternatives char + _
print(re.sub('(.) spam', 'spam\1', 'x spam, y spam')) # group back ref (or r'')
def mapper(matchobj):
return 'spam' + matchobj.group(1)
print(re.sub('(.) spam', mapper, 'x spam, y spam')) # mapping function
In the first test, all characters in the set are replaced; in the second, they must be followed
by an underscore. The last two tests illustrate more advanced group back-references
and mapping functions in the replacement. Note the \1 required to escape \1 for
Python’s string rules; r'spam\1' would work just as well. See also the earlier interactive
tests in the section for additional substitution and splitting examples:
C:\...\PP4E\Lang> python re-subst.py
X*X*X*X*X*X*
XA-X*XB-X*XC-X*
spamx, spamy
spamx, spamyScanning C Header Files for Patterns
To wrap up, let’s turn to a more realistic example: the script in Example 19-6 puts these
pattern operators to more practical use. It uses regular expressions to find #define and
#include lines in C header files and extract their components. The generality of the
patterns makes them detect a variety of line formats; pattern groups (the parts in pa-
rentheses) are used to extract matched substrings from a line after a match.
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