How to match integers in NLTK CFG?

Question

If I want to define a grammar in which one of the tokens will match an integer, how can i achieve it using nltk\'s string CFG?

For example -

S -> SK S         


        

Answer 1

Create a number phrase as such:

import nltk

groucho_grammar = nltk.CFG.fromstring("""
S -> NP VP
PP -> P NP
NP -> Det N | Det N PP | 'I' | NUM N
VP -> V NP | VP PP
Det -> 'an' | 'my'
N -> 'elephant' | 'pajamas' | 'elephants'
V -> 'shot'
P -> 'in'
NUM -> '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' | '10'
""")

sent = 'I shot 3 elephants'.split()
parser = nltk.ChartParser(groucho_grammar)
for tree in parser.parse(sent):
    print(tree)

[out]:

(S (NP I) (VP (V shot) (NP (NUM 3) (N elephants))))

But note that that can only handle single digit number. So let's try compressing integers into a single token-type, e.g. '#NUM#':

import nltk

groucho_grammar = nltk.CFG.fromstring("""
S -> NP VP
PP -> P NP
NP -> Det N | Det N PP | 'I' | NUM N
VP -> V NP | VP PP
Det -> 'an' | 'my'
N -> 'elephant' | 'pajamas' | 'elephants'
V -> 'shot'
P -> 'in'
NUM -> '#NUM#'
""")

sent = 'I shot 333 elephants'.split()
sent = ['#NUM#' if i.isdigit() else i for i in sent]

parser = nltk.ChartParser(groucho_grammar)
for tree in parser.parse(sent):
    print(tree)

[out]:

(S (NP I) (VP (V shot) (NP (NUM #NUM#) (N elephants))))

To put the numbers back, try:

import nltk

groucho_grammar = nltk.CFG.fromstring("""
S -> NP VP
PP -> P NP
NP -> Det N | Det N PP | 'I' | NUM N
VP -> V NP | VP PP
Det -> 'an' | 'my'
N -> 'elephant' | 'pajamas' | 'elephants'
V -> 'shot'
P -> 'in'
NUM -> '#NUM#'
""")

original_sent = 'I shot 333 elephants'.split()
sent = ['#NUM#' if i.isdigit() else i for i in original_sent]
numbers = [i for i in original_sent if i.isdigit()]

parser = nltk.ChartParser(groucho_grammar)
for tree in parser.parse(sent):
    treestr = str(tree)
    for n in numbers:
        treestr = treestr.replace('#NUM#', n, 1)
    print(treestr)

[out]:

(S (NP I) (VP (V shot) (NP (NUM 333) (N elephants))))