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    <p>A recursive descent parser is a simple form of parser that can parse many grammars. While the entire theory of parsing is too large for a stack-overflow answer, the most common approach to parsing involves two steps: first, tokenisation, which extracts subwords of your string (here, probably words like 'Root', and 'ABC', or brackets like '(' and ')'), and then parsing using recursive functions.</p> <p>This code parses input (like your example), producing a so-called parse tree, and also has a function 'show_children' which takes the parse tree, and produces the children view of the expression as your question asked.</p> <pre><code>import re class ParseError(Exception): pass # Tokenize a string. # Tokens yielded are of the form (type, string) # Possible values for 'type' are '(', ')' and 'WORD' def tokenize(s): toks = re.compile(' +|[A-Za-z]+|[()]') for match in toks.finditer(s): s = match.group(0) if s[0] == ' ': continue if s[0] in '()': yield (s, s) else: yield ('WORD', s) # Parse once we're inside an opening bracket. def parse_inner(toks): ty, name = next(toks) if ty != 'WORD': raise ParseError children = [] while True: ty, s = next(toks) if ty == '(': children.append(parse_inner(toks)) elif ty == ')': return (name, children) # Parse this grammar: # ROOT ::= '(' INNER # INNER ::= WORD ROOT* ')' # WORD ::= [A-Za-z]+ def parse_root(toks): ty, _ = next(toks) if ty != '(': raise ParseError return parse_inner(toks) def show_children(tree): name, children = tree if not children: return print '%s -&gt; %s' % (name, ' '.join(child[0] for child in children)) for child in children: show_children(child) example = '( Root ( AB ( ABC ) ( CBA ) ) ( CD ( CDE ) ( FGH ) ) )' show_children(parse_root(tokenize(example))) </code></pre>
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    1. POHow to parse parenthetical trees in python?
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    1. USPaul Hankin
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    1. POHow to parse parenthetical trees in python?
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    1. COOne of the flaws of Python (at least CPython) is a relatively shallow maximum stack depth. This is nice and clean code, but it relies on the Python stack, so it will be limited in how deep of a tree it can parse. Other solutions, such as the one I posted, can handle very deep trees. If this solved the problem I'd leave it as-is because I like the simplicity, but this could be rewritten to use a `list` as a stack to keep the state, and then it would be able to parse very deep trees as well.
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