Natural Language Processing

Developing a natural language parsing model involves tasks such as tokenization, parsing, and possibly dealing with ambiguous or erroneous inputs.

Natural language processing is a complex field that goes beyond just programming. It involves knowledge of linguistics, and many parsing tasks can be difficult due to the ambiguity and complexity of natural languages. You may want to look into existing tools and libraries specifically designed for natural language processing, like NLTK or SpaCy for Python, in addition to the general-purpose libraries that Boost offers.

Libraries

  • Boost.Spirit: This is Boost’s library for creating parsers and output generation. It includes support for creating grammars, which can be used to define the structure of sentences in English, or other language. You can use it to tokenize and parse input text according to your grammars.

  • Boost.Regex: For some simpler parsing tasks, regular expressions can be sufficient and easier to use than full-blown parsing libraries. You could use Boost.Regex to match specific patterns in your input text, like specific words or phrases, word boundaries, etc.

  • Boost.String_Algo: Provides various string manipulation algorithms, such as splitting strings, trimming whitespace, or replacing substrings. These can be useful in preprocessing text before parsing it.

  • Boost.Optional and Boost.Variant: These libraries can be helpful in representing the result of a parsing operation, which could be a successful parse (yielding a specific result), an error, or an ambiguous parse (yielding multiple possible results).

  • Boost.MultiIndex: Provides data structures that can be indexed in multiple ways. It can be useful for storing and retrieving information about words or phrases in your language model, such as part of speech, frequency, or possible meanings.

  • Boost.PropertyTree or Boost.Json: These libraries can be useful for handling input and output, such as reading configuration files or producing structured output.

Natural Language Processing Applications

Natural language parsing (NLP) is a foundational technology that powers many more applications and is a hot area of research and development.

  • Search Engines: Search engines like Google extensively use natural language processing to understand the intent behind user queries and provide accurate and relevant search results.

  • Machine Translation: Services like Google Translate and DeepL rely on natural language parsing for their operation. These services can now translate text between many different languages with surprising accuracy.

  • Speech Recognition: Virtual assistants like Apple’s Siri, Amazon’s Alexa, Google Assistant, and Microsoft’s Cortana, use natural language parsing to understand voice commands from users.

  • Text Summarization: This is used in a variety of contexts, from generating summaries of news articles, scientific papers, to creating summaries of lengthy documents for quicker reading.

  • Sentiment Analysis: Businesses use NLP to gauge public opinion about their products and services by analyzing social media posts, customer reviews, and other user-generated content.

  • Chatbots and Virtual Assistants: NLP is key to the operation of automated chatbots, which are used for everything from customer service to mental health support.

  • Information Extraction: Companies use NLP to extract specific information from unstructured text, like dates, names, or specific keywords.

  • Text-to-Speech (TTS): Services that convert written text into spoken words often use natural language processing to ensure proper pronunciation, intonation, and timing.

  • Grammar Checkers: Tools like Grammarly use natural language processing to correct grammar, punctuation, and style in written text.

  • Content Recommendation: Platforms like YouTube or Netflix use NLP to analyze the content and provide more accurate and personalized recommendations.

Simple English Parsing Sample

Say we wanted to parse a subset of the English language, only sentence of the form: The <adjective> <noun> <verb>. Example sentences could be "The quick fox jumps." or "The lazy dog sleeps."

If the input matches this grammar, the following parser will accept it. Otherwise, it rejects the input.

#include <iostream>
#include <string>
#include <boost/spirit/include/qi.hpp>

namespace qi = boost::spirit::qi;

bool parseSentence(const std::string& input) {

    // Grammar: The <adjective> <noun> <verb>.
    qi::rule<std::string::const_iterator, std::string()> word = +qi::alpha;
    qi::rule<std::string::const_iterator, std::string()> article = qi::lit("The");
    qi::rule<std::string::const_iterator, std::string()> adjective = word;
    qi::rule<std::string::const_iterator, std::string()> noun = word;
    qi::rule<std::string::const_iterator, std::string()> verb = word;

    // Full sentence rule
    qi::rule<std::string::const_iterator, std::string()> sentence =
        article >> adjective >> noun >> verb >> qi::lit('.');

    auto begin = input.begin(), end = input.end();
    bool success = qi::parse(begin, end, sentence);

    return success && (begin == end);  // Ensure full input is consumed
}

int main() {
    std::string input;
    std::cout << "Enter a sentence (format: The <adjective> <noun> <verb>.)\n";
    std::getline(std::cin, input);

    if (parseSentence(input)) {
        std::cout << "Valid sentence!\n";
    } else {
        std::cout << "Invalid sentence.\n";
    }

    return 0;
}
cpp

The following shows a successful parse:

Enter a sentence (format: The <adjective> <noun> <verb>.)
The happy cat purrs.
Valid sentence!
bash

And the following shows an unsuccessful parse:

Enter a sentence (format: The <adjective> <noun> <verb>.)
A small dog runs.
Invalid sentence.
bash

Our subset is clearly very limited, as simply replacing the word "The" with "A" results in an error.

Add a Dictionary of Valid Words

We can extend the simple example to use a dictionary of valid words, allow multiple adjectives, and use Boost.String_Algo for some string processing tasks (trimming spaces, converting to lowercase).

#include <iostream>
#include <string>
#include <unordered_set>
#include <boost/spirit/include/qi.hpp>
#include <boost/algorithm/string.hpp>

namespace qi = boost::spirit::qi;

// Dictionary of valid words
const std::unordered_set<std::string> valid_adjectives = {"quick", "lazy", "happy", "small", "big", "brown"};
const std::unordered_set<std::string> valid_nouns = {"fox", "dog", "cat", "rabbit"};
const std::unordered_set<std::string> valid_verbs = {"jumps", "sleeps", "runs", "eats"};

bool is_valid_word(const std::string& word, const std::unordered_set<std::string>& dictionary) {
    return dictionary.find(word) != dictionary.end();
}

// Parses: "The <adjective> <adjective>... <noun> <verb>."
bool parseSentence(const std::string& input) {
    std::string sentence = input;

    // Use Boost.StringAlgo to trim and convert to lowercase
    boost::algorithm::trim(sentence);
    boost::algorithm::to_lower(sentence);

    // Define grammar
    qi::rule<std::string::const_iterator, std::string()> word = +qi::alpha;
    qi::rule<std::string::const_iterator, std::string()> article = qi::lit("the");

    // Multiple adjectives allowed
    std::vector<std::string> adjectives;
    auto adjective_parser = +word[([&](auto& ctx) { adjectives.push_back(_attr(ctx)); })];

    std::string noun, verb;
    auto noun_parser = word[([&](auto& ctx) { noun = _attr(ctx); })];
    auto verb_parser = word[([&](auto& ctx) { verb = _attr(ctx); })];

    qi::rule<std::string::const_iterator, std::string()> sentence_parser =
        article >> adjective_parser >> noun_parser >> verb_parser >> qi::lit('.');

    // Parse input
    auto begin = sentence.begin(), end = sentence.end();
    bool success = qi::parse(begin, end, sentence_parser) && (begin == end);

    // Validate words using dictionaries
    if (!success) return false;
    if (!is_valid_word(noun, valid_nouns) || !is_valid_word(verb, valid_verbs)) return false;
    for (const auto& adj : adjectives) {
        if (!is_valid_word(adj, valid_adjectives)) return false;
    }

    return true;
}

int main() {
    std::string input;
    std::cout << "Enter a sentence (e.g., The big brown fox jumps.):\n";
    std::getline(std::cin, input);

    if (parseSentence(input)) {
        std::cout << "✅ Valid sentence!\n";
    } else {
        std::cout << "❌ Invalid sentence.\n";
    }

    return 0;
}
cpp

The following shows a successful parse:

Enter a sentence (e.g., The big brown fox jumps.):
The big brown fox jumps.
✅ Valid sentence!
bash

And the following shows an unsuccessful parse:

Enter a sentence (e.g., The big brown fox jumps.):
The huge blue dragon flies.
❌ Invalid sentence.
bash

Add Detailed Error Reporting

#include <iostream>
#include <string>
#include <unordered_set>
#include <vector>
#include <boost/spirit/include/qi.hpp>
#include <boost/algorithm/string.hpp>

namespace qi = boost::spirit::qi;

// Dictionary of valid words
const std::unordered_set<std::string> valid_adjectives = {"quick", "lazy", "happy", "small", "big", "brown"};
const std::unordered_set<std::string> valid_nouns = {"fox", "dog", "cat", "rabbit"};
const std::unordered_set<std::string> valid_verbs = {"jumps", "sleeps", "runs", "eats"};

// Function to check if a word is in a dictionary
bool is_valid_word(const std::string& word, const std::unordered_set<std::string>& dictionary) {
    return dictionary.find(word) != dictionary.end();
}

// Parses: "The <adjective> <adjective>... <noun> <verb>."
bool parseSentence(const std::string& input, std::string& error_message) {
    std::string sentence = input;

    // Use Boost.StringAlgo to trim and convert to lowercase
    boost::algorithm::trim(sentence);
    boost::algorithm::to_lower(sentence);

    // Define grammar
    qi::rule<std::string::const_iterator, std::string()> word = +qi::alpha;
    qi::rule<std::string::const_iterator, std::string()> article = qi::lit("the");

    std::vector<std::string> adjectives;
    std::string noun, verb;

    // Adjective parser
    auto adjective_parser = *(word[([&](auto& ctx) { adjectives.push_back(_attr(ctx)); })]);

    // Noun parser
    auto noun_parser = word[([&](auto& ctx) { noun = _attr(ctx); })];

    // Verb parser
    auto verb_parser = word[([&](auto& ctx) { verb = _attr(ctx); })];

    // Full sentence parser
    qi::rule<std::string::const_iterator, std::string()> sentence_parser =
        article >> adjective_parser >> noun_parser >> verb_parser >> qi::lit('.');

    // Parse input
    auto begin = sentence.begin(), end = sentence.end();
    bool success = qi::parse(begin, end, sentence_parser) && (begin == end);

    // Check syntax errors
    if (!success) {
        error_message = "❌ Syntax error: Sentence structure should be 'The <adjective> <adjective>... <noun> <verb>.'";
        return false;
    }

    // Check dictionary validation
    for (const auto& adj : adjectives) {
        if (!is_valid_word(adj, valid_adjectives)) {
            error_message = "❌ Invalid word: '" + adj + "' is not a recognized adjective.";
            return false;
        }
    }
    if (!is_valid_word(noun, valid_nouns)) {
        error_message = "❌ Invalid word: '" + noun + "' is not a recognized noun.";
        return false;
    }
    if (!is_valid_word(verb, valid_verbs)) {
        error_message = "❌ Invalid word: '" + verb + "' is not a recognized verb.";
        return false;
    }

    return true;
}

int main() {
    std::string input;
    std::cout << "Enter a sentence (e.g., The big brown fox jumps.):\n";
    std::getline(std::cin, input);

    std::string error_message;
    if (parseSentence(input, error_message)) {
        std::cout << "✅ Valid sentence!\n";
    } else {
        std::cout << error_message << "\n";
    }

    return 0;
}
cpp

The following shows a successful parse:

Enter a sentence (e.g., The big brown fox jumps.):
The big brown fox jumps.
✅ Valid sentence!
bash

And the following shows several unsuccessful parses:

Enter a sentence (e.g., The big brown fox jumps.):
The huge blue dragon flies.
❌ Invalid sentence.

Enter a sentence (e.g., The big brown fox jumps.):
The gigantic brown fox jumps.
❌ Invalid word: 'gigantic' is not a recognized adjective.

Enter a sentence (e.g., The big brown fox jumps.):
The big brown dragon jumps.
❌ Invalid word: 'dragon' is not a recognized noun.

Enter a sentence (e.g., The big brown fox jumps.):
The big brown fox flies.
❌ Invalid word: 'flies' is not a recognized verb.
bash

You will notice how adding more features to a natural language parser starts to considerably increase the code length. This is a normal feature of language parsing - a lot of code can be required to cover all the options of something as flexible as language. For an example of a simpler approach to parsing well-formatted input, refer to the sample code in Text Processing.

See Also

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