Use regular expressions to transform file names

A particularly powerfule file mapper deserves more detailed description: It's the regular expression file mapper.

This file mapper uses regular expressions to identify certain pieces in the input file name. This piece, or these pieces are replaced with a replacement pattern.

Let's first design a configuration snippet, which would transform all files in the directory src/main/xml by applying the stylesheet src/main/stylesheet.xsl. The created files would have the extension .fo, rather than .xml. (We could very well achieve the same by using the FileExtensionMapper, as demonstrated in this example.)

  <build>
    <plugins>
      ...
      <plugin>
        <groupId>org.codehaus.mojo</groupId>
        <artifactId>xml-maven-plugin</artifactId>
        <executions>
          <execution>
            <goals>
              <goal>transform</goal>
            </goals>
          </execution>
        </executions>
        <configuration>
          <transformationSets>
            <transformationSet>
              <dir>src/main/xml</dir>
              <stylesheet>src/main/stylesheet.xsl</stylesheet>
              <fileMappers>
                <fileMapper implementation="org.codehaus.plexus.components.io.filemappers.RegExpFileMapper">
                  <pattern>\.gif$</pattern>
                  <replacement>.jpg</replacement>
                </fileMapper>
              </fileMappers>
            </transformationSet>
          </transformationSets>
        </configuration>
      </plugin>
      ...
    </plugins>
  </build>

Let's have a look at some other examples. We omit most of the details, except for the pattern and the replacement, which are the important parts:

    <fileMapper implementation="org.codehaus.plexus.components.io.filemappers.RegExpFileMapper">
      <pattern>^(.*)\.gif$</pattern>
      <replacement>$1.jpg</replacement>
    </fileMapper>

This example is identical with the previous. However, it demonstrates the use of capture groups. The contents of (.*) are captured and inserted by using the group term $1. You might have multiple groups and use the terms $2, $3, and so on.

    <fileMapper implementation="org.codehaus.plexus.components.io.filemappers.RegExpFileMapper">
      <pattern>a</pattern>
      <replacement>b</replacement>
    </fileMapper>

This example replaces the first a with a b. Nothing special here, except that you might like to replace all a's instead. Here's how to do it:

    <fileMapper implementation="org.codehaus.plexus.components.io.filemappers.RegExpFileMapper">
      <pattern>a</pattern>
      <replacement>b</replacement>
      <replaceAll>true</replaceAll>
    </fileMapper>