Nested Regex Conditionals

Paul McGuire ptmcg at austin.rr.com
Tue Aug 23 23:39:39 CEST 2005


This works (note relocated right paren at (?(second) ):

#!/usr/bin/env python
import re
original_pattern = re.compile(r"""
    (?P<first>(first))
      (?(first)
        (?P<second>(second))
      )
      (?(second)
        (?P<third>(third))
      )
      """, re.VERBOSE)
pattern = re.compile(r"""
    (?P<first>(first))
      (?(first)
        (?P<second>(second))
      )
      (?(second))
        (?P<third>(third))
      """, re.VERBOSE)

string = 'firstsecondthird'
match = re.match(pattern,string)
print match.group('first','second','third')


Prints out:
('first', 'second', 'third')


The pyparsing alternative looks like:

first = Literal("first").setResultsName("first")
second = Literal("second").setResultsName("second")
third = Literal("third").setResultsName("third")

# define line
line = first + second + third

print line.parseString("firstsecondthird")

Prints out:
['first', 'second', 'third']

-- Paul




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