NokogiriEdit
Nokogiri is a widely used Ruby gem that provides robust facilities for parsing and manipulating XML and HTML documents by leveraging the efficiency of the C libraries libxml2 and libxslt. It gives Ruby developers a practical, battle-tested interface to read, search, transform, and serialize document data. With support for both XML and HTML, Nokogiri is a core tool in data extraction, content normalization, and integration tasks within the Ruby ecosystem, including applications built on Rails and other frameworks. It exposes powerful search mechanisms through XPath and CSS selectors, while also offering streaming interfaces to handle large documents without exhausting memory.
From a software development perspective, Nokogiri is designed to be fast, reliable, and interoperable with established web standards. It wraps the native parsing engines in libxml2 and libxslt and exposes a Ruby-friendly API through modules like Nokogiri::XML and Nokogiri::HTML. This alignment with mature libraries helps ensure compatibility with a wide range of inputs, encodings, and document structures, while still benefiting from Ruby’s expressive syntax and tooling such as RubyGems for distribution and dependency management.
History and design
Nokogiri emerged to address a gap in the Ruby ecosystem for a high-performance, standards-compliant parser that could handle both well-formed XML and the often-messy markup encountered in the wild. By building on libxml2 for parsing and libxslt for transformation, the project could combine speed with feature completeness, including support for XPath queries and CSS selectors.
A key design choice is the separation of concerns between HTML and XML processing. Nokogiri provides Nokogiri::HTML for HTML parsing, which tends to be forgiving of malformed markup, and Nokogiri::XML for XML parsing, which enforces stricter well-formedness rules. This mirrors the practical needs of developers who ingest data from diverse sources: web pages, RSS/Atom feeds, configuration files, and more. The library also exposes streaming interfaces such as Nokogiri::XML::Reader (pull parsing) and, in SAX-like form via Nokogiri::XML::SAX, to enable scalable processing of large inputs.
Nokogiri’s development has been sustained by a community of contributors within the Ruby ecosystem, with releases coordinated through RubyGems and compatibility considerations for different versions of libxml2 and libxslt. The project emphasizes a stable, Ruby-friendly API that still leverages the performance advantages of its underlying C libraries.
Features
Parsing and serialization: Nokogiri can read documents from strings, files, or IO streams and produce serialized output in HTML or XML form. It supports common document encodings and preserves or normalizes namespaces as needed.
Dual parsing flavors: Nokogiri::HTML handles HTML with forgiving parsing behavior, suitable for web scraping and content extraction, while Nokogiri::XML provides stricter XML parsing and validation as appropriate for XML data interchange.
Rich search capabilities: Users can locate nodes via XPath expressions or via CSS selectors (a familiar pattern for web developers) and iterate over matched results. This flexibility makes it easy to write concise queries that extract links, metadata, or structured data from documents.
Document manipulation and construction: The API permits creating, modifying, and removing nodes, attributes, and namespaces, enabling tasks such as injecting content, normalizing markup, or rebuilding documents for downstream processing.
Streaming and memory efficiency: For large inputs, Nokogiri offers streaming interfaces to process data incrementally, avoiding the memory footprint of loading entire documents into a single in-memory tree.
Safety and correctness: Given the security considerations around parsing arbitrary input, Nokogiri provides options and sensible defaults to mitigate risks such as external entity expansion and harmful markup. Developers are advised to be mindful of document sources and to apply appropriate parsing options when handling untrusted data.
Interoperability with other libraries: As a bridge to the standards implemented by libxml2 and libxslt, Nokogiri integrates well with existing tooling for XML transformations (XSLT) and schema-driven validation workflows, while remaining approachable from Ruby code.
Usage patterns and ecosystem
Nokogiri is commonly used in web scraping, data integration, and content normalization workflows. Typical patterns include parsing an HTML page to extract links or meta tags, transforming XML data via XSLT, or validating and reserializing documents for storage or transmission. In the Ruby ecosystem, it often serves as a companion to frameworks like Rails and to libraries for HTTP requests, HTML parsing, and data pipelines.
Because Nokogiri is a native extension that wraps well-established C libraries, it tends to outperform pure Ruby parsers such as REXML in both speed and robustness. At the same time, its API remains accessible to developers who are comfortable with Ruby’s idioms, making it a practical choice for both small scripts and large-scale data processing tasks.
Common considerations when working with Nokogiri include choosing the right parsing mode (HTML vs XML), handling encodings correctly, and being mindful of security implications when parsing untrusted inputs. Developers often combine Nokogiri with other parts of the Ruby toolchain—such as RubyGems for dependency management and Rails for web applications—to build end-to-end data ingestion and presentation solutions.