nltk.tokenize.sexpr module

S-Expression Tokenizer

SExprTokenizer is used to find parenthesized expressions in a string. In particular, it divides a string into a sequence of substrings that are either parenthesized expressions (including any nested parenthesized expressions), or other whitespace-separated tokens.

>>> from nltk.tokenize import SExprTokenizer
>>> SExprTokenizer().tokenize('(a b (c d)) e f (g)')
['(a b (c d))', 'e', 'f', '(g)']

By default, SExprTokenizer will raise a ValueError exception if used to tokenize an expression with non-matching parentheses:

>>> SExprTokenizer().tokenize('c) d) e (f (g')
Traceback (most recent call last):
  ...
ValueError: Un-matched close paren at char 1

The strict argument can be set to False to allow for non-matching parentheses. Any unmatched close parentheses will be listed as their own s-expression; and the last partial sexpr with unmatched open parentheses will be listed as its own sexpr:

>>> SExprTokenizer(strict=False).tokenize('c) d) e (f (g')
['c', ')', 'd', ')', 'e', '(f (g']

The characters used for open and close parentheses may be customized using the parens argument to the SExprTokenizer constructor:

>>> SExprTokenizer(parens='{}').tokenize('{a b {c d}} e f {g}')
['{a b {c d}}', 'e', 'f', '{g}']

The s-expression tokenizer is also available as a function:

>>> from nltk.tokenize import sexpr_tokenize
>>> sexpr_tokenize('(a b (c d)) e f (g)')
['(a b (c d))', 'e', 'f', '(g)']
class nltk.tokenize.sexpr.SExprTokenizer[source]

Bases: nltk.tokenize.api.TokenizerI

A tokenizer that divides strings into s-expressions. An s-expresion can be either:

  • a parenthesized expression, including any nested parenthesized expressions, or

  • a sequence of non-whitespace non-parenthesis characters.

For example, the string (a (b c)) d e (f) consists of four s-expressions: (a (b c)), d, e, and (f).

By default, the characters ( and ) are treated as open and close parentheses, but alternative strings may be specified.

Parameters
  • parens (str or list) – A two-element sequence specifying the open and close parentheses that should be used to find sexprs. This will typically be either a two-character string, or a list of two strings.

  • strict – If true, then raise an exception when tokenizing an ill-formed sexpr.

__init__(parens='()', strict=True)[source]
tokenize(text)[source]

Return a list of s-expressions extracted from text. For example:

>>> SExprTokenizer().tokenize('(a b (c d)) e f (g)')
['(a b (c d))', 'e', 'f', '(g)']

All parentheses are assumed to mark s-expressions. (No special processing is done to exclude parentheses that occur inside strings, or following backslash characters.)

If the given expression contains non-matching parentheses, then the behavior of the tokenizer depends on the strict parameter to the constructor. If strict is True, then raise a ValueError. If strict is False, then any unmatched close parentheses will be listed as their own s-expression; and the last partial s-expression with unmatched open parentheses will be listed as its own s-expression:

>>> SExprTokenizer(strict=False).tokenize('c) d) e (f (g')
['c', ')', 'd', ')', 'e', '(f (g']
Parameters

text (str or iter(str)) – the string to be tokenized

Return type

iter(str)

nltk.tokenize.sexpr.sexpr_tokenize(text)

Return a list of s-expressions extracted from text. For example:

>>> SExprTokenizer().tokenize('(a b (c d)) e f (g)')
['(a b (c d))', 'e', 'f', '(g)']

All parentheses are assumed to mark s-expressions. (No special processing is done to exclude parentheses that occur inside strings, or following backslash characters.)

If the given expression contains non-matching parentheses, then the behavior of the tokenizer depends on the strict parameter to the constructor. If strict is True, then raise a ValueError. If strict is False, then any unmatched close parentheses will be listed as their own s-expression; and the last partial s-expression with unmatched open parentheses will be listed as its own s-expression:

>>> SExprTokenizer(strict=False).tokenize('c) d) e (f (g')
['c', ')', 'd', ')', 'e', '(f (g']
Parameters

text (str or iter(str)) – the string to be tokenized

Return type

iter(str)