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+#
+# simpleArith.py
+#
+# Example of defining an arithmetic expression parser using
+# the operatorPrecedence helper method in pyparsing.
+#
+# Copyright 2006, by Paul McGuire
+#
+
+from pyparsing import *
+
+integer = Word(nums).setParseAction(lambda t:int(t[0]))
+variable = Word(alphas,exact=1)
+operand = integer | variable
+
+expop = Literal('^')
+signop = oneOf('+ -')
+multop = oneOf('* /')
+plusop = oneOf('+ -')
+factop = Literal('!')
+
+# To use the operatorPrecedence helper:
+#   1.  Define the "atom" operand term of the grammar.
+#       For this simple grammar, the smallest operand is either
+#       and integer or a variable.  This will be the first argument
+#       to the operatorPrecedence method.
+#   2.  Define a list of tuples for each level of operator
+#       precendence.  Each tuple is of the form
+#       (opExpr, numTerms, rightLeftAssoc, parseAction), where
+#       - opExpr is the pyparsing expression for the operator;
+#          may also be a string, which will be converted to a Literal
+#       - numTerms is the number of terms for this operator (must
+#          be 1 or 2)
+#       - rightLeftAssoc is the indicator whether the operator is
+#          right or left associative, using the pyparsing-defined
+#          constants opAssoc.RIGHT and opAssoc.LEFT.
+#       - parseAction is the parse action to be associated with 
+#          expressions matching this operator expression (the
+#          parse action tuple member may be omitted)
+#   3.  Call operatorPrecedence passing the operand expression and
+#       the operator precedence list, and save the returned value
+#       as the generated pyparsing expression.  You can then use
+#       this expression to parse input strings, or incorporate it
+#       into a larger, more complex grammar.
+#       
+expr = operatorPrecedence( operand,
+    [("!", 1, opAssoc.LEFT),
+     ("^", 2, opAssoc.RIGHT),
+     (signop, 1, opAssoc.RIGHT),
+     (multop, 2, opAssoc.LEFT),
+     (plusop, 2, opAssoc.LEFT),]
+    )
+
+test = ["9 + 2 + 3",
+        "9 + 2 * 3",
+        "(9 + 2) * 3",
+        "(9 + -2) * 3",
+        "(9 + -2) * 3^2^2",
+        "(9! + -2) * 3^2^2",
+        "M*X + B",
+        "M*(X + B)",
+        "1+2*-3^4*5+-+-6",]
+for t in test:
+    print t
+    print expr.parseString(t)
+    print 
+