hamcrest-text-patterns

A library for writing readable, composable regular expression matchers that integrates cleanly with Hamcrest.

Features

• Easier to read (although more long-winded) than regular expressions.
• Named capture groups: captured text is identified by name, not by the index of the group.
• Composable: patterns can be easily combined into more complex patterns without worrying about breaking regex syntax or changing group identifiers.
• Refactoring friendly: patterns can be refactored with your favourite IDE.

Parsing

Here's a simple example of using Hamcrest Text Patterns to parse date ranges, such as "12 Apr 1994 - 16 Mar 2005".

We can define components of the final text pattern using the factory methods defined in the Patterns class:

PatternComponent month = either(
  "jan", "feb", "mar", "apr", "may", "jun", 
  "jul", "aug", "sep", "oct", "nov", "dec");

PatternComponent digit = anyCharacterInCategory("Digit");

PatternComponent date = separatedBy(whitespace(),
  capture("day", exactly(2, digit)), 
  capture("month", caseInsensitive(month)), 
  capture("year", exactly(4, digit)));

We can combine these into a PatternMatcher for matching date ranges:

PatternMatcher dateRange = new PatternMatcher(separatedBy(whitespace(),
  capture("from", date), "-", capture("to", date)));

We can now parse date ranges from input text and extract the dates using the names of the capture groups we defined in the dateRange pattern: "from" and "to".

String input = "31 Dec 2003 - 16 aug 2008";

Parse parse = dateRange.parse(input);

assertThat(parse.get("from"), equalTo("31 Dec 2003"));
assertThat(parse.get("to"), equalTo("16 aug 2008"));

And, because we defined capture groups in our date pattern, we can extract the parts of the "from" and "to" dates:

assertThat(parse.get("from.day"), equalTo("31"));
assertThat(parse.get("from.month"), equalTo("Dec"));
assertThat(parse.get("from.year"), equalTo("2003"));

assertThat(parse.get("to.day"), equalTo("16"));
assertThat(parse.get("to.month"), equalTo("aug"));
assertThat(parse.get("to.year"), equalTo("2008"));

Matching

A PatternMatcher is a Hamcrest matcher, and so text patterns can be used with libraries that build upon Hamcrest, such as JUnit or jMock. For example:

String input = "31 Dec 2003 - 16 aug 2008";
assertThat(input, dateRange);

String badInput = "31 12 2003 - 16 08 2008";
assertThat(badInput, not(dateRange));

Writing New Pattern Components

The library does not cover all the Java regex functionality yet, so you may need to extend the library to add a feature you need.

For example, lets write a pattern component that matches ASCII control characters.

We must define a class that implements the PatternComponent interface.

public class AsciiControlCharacter implements PatternComponent {
  ...
}

Then, we must implement the buildRegex method to append the regex syntax for the pattern component to the StringBuilder.

public class AsciiControlCharacter implements PatternComponent {
  public void buildRegex(StringBuilder builder, GroupNamespace groups) {
    builder.append("\\p{Cntrl}");
  }
}

(We can ignore the groups parameter. That's used by the capture(...) and valueOf(...) pattern components to identify capture groups by name.)

Finally, we should write a static function to make patterns that use this component easy to read. For example, we could put factory functions for all the pattern components we need for our project in a single class and statically import them into the classes that use them.

public class ProjectSpecificPatterns {
  public static PatternComponent asciiControlCharacter() {
    return new AsciiControlCharacter();
  }
}

Maven

To use hamcrest-text-patterns with Maven 2/3, you must add the following repository to your project's pom.xml file:

<repository>
  <id>hamcrest-text-patterns-repo</id>
  <name>hamcrest-text-patterns repository on GitHub</name>
  <url>http://wcmatthysen.github.com/hamcrest-text-patterns/repository/</url>
</repository>

Then, you can add the following dependency:

<dependency>
  <groupId>org.hamcrest</groupId>
  <artifactId>hamcrest-text-patterns</artifactId>
  <version>0.0.1</version>
</dependency>

License

BSD License

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