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Diffstat (limited to 'examples/left_recursion.py')
-rw-r--r-- | examples/left_recursion.py | 45 |
1 files changed, 45 insertions, 0 deletions
diff --git a/examples/left_recursion.py b/examples/left_recursion.py new file mode 100644 index 0000000..f3977dc --- /dev/null +++ b/examples/left_recursion.py @@ -0,0 +1,45 @@ +# +# left_recursion.py +# +# Example code illustrating use of left-recursion in Pyparsing. +# +import pyparsing as pp + +# comment out this line to see the effects without LR parsing enabled +pp.ParserElement.enableLeftRecursion() + +item_list = pp.Forward() + +# a common left-recursion definition +# define a list of items as 'list + item | item' +# BNF: +# item_list := item_list item | item +# item := word of alphas +item = pp.Word(pp.alphas) +item_list <<= item_list + item | item + +item_list.runTests("""\ + To parse or not to parse that is the question + """) + +# Define a parser for an expression that can be an identifier, a quoted string, or a +# function call that starts with an expression +# BNF: +# expr := function_call | name | string | '(' expr ')' +# function_call := expr '(' expr,... ')' +# name := Python identifier +# string := a quoted string +# from https://stackoverflow.com/questions/32809389/parse-python-code-using-pyparsing/32822575#32822575 + +LPAR, RPAR = map(pp.Suppress, "()") +expr = pp.Forward() +string = pp.quotedString +function_call = expr + pp.Group(LPAR + pp.Optional(pp.delimitedList(expr)) + RPAR) +name = pp.Word(pp.alphas + '_', pp.alphanums + '_') +# left recursion - call starts with an expr +expr <<= function_call | string | name | pp.Group(LPAR + expr + RPAR) + +expr.runTests("""\ + print("Hello, World!") + (lookup_function("fprintf"))(stderr, "Hello, World!") + """, fullDump=False) |