4.4 Relax NG Grammar

With a little bit of experience it is not that difficult to write and understand Phing build files since the XML format in itself tends to be quite verbose. However, it can become a bit tedious and the large (and growing) amount of built-in tasks and filters can sometimes make it difficult to remember the exact syntax of all the available features.

To help with this the Phing distribution contains a Relax NG Grammar (REgular LAnguage for XML Next Generation, http://www.relaxng.org/) file that describes the (formal) syntax of the build files. This grammar can be used to validate build files. However, the most beneficial use of the grammar is together with a schema aware XML editor. Such an editor can make auto-completion based on the grammar. This feature makes writing complex build files significantly easier since it is usually enough to enter the first letter of an element to have the rest of the element written automatically as well as any compulsory attributes.

Most XML editors can be told to what schema (or model) to use for validation and auto-completion by adding a specification in the beginning of the XML file. For example, the following two lines in the beginning of an XML file would do (of course the exact path to the grammar will depend on your system setup)

<?xml version="1.0" encoding="UTF-8"?>
<?xml-model xlink:href="/usr/share/php5/PEAR/data/phing/etc/phing-grammar.rng"
            schematypens="http://relaxng.org/ns/structure/1.0" ?>

Using auto-completion will make it substantially easier to edit large build files. Please note that since the phing-grammar does not have an official designation we must use the absolute filename to specify the grammar (instead of a canonical URI that is resolved by the systems XML-catalogue).

This grammar is available (as a plain text file) in the distribution at: /etc/phing-grammar.rng

Since we do not want to neither endorse nor forget any particular XML editor with this capability we do not make available such a list of editors. Instead, spending a few minutes with Google searching for XML-editors is bound to find a number of editors with this capability.

If you wish to validate your Phing build file, there are numerous options. Links to various validation tools and XML editors are available at the RELAX NG home page, http://www.relaxng.org/. The command line tool xmllint that comes with libxml2 is also able to validate a given XML file against the supplied grammar.

For example, to use xmllint to validate a Phing build file the following command line could be used:

$ xmllint -noout -relaxng phing-grammar.rng build.xml
build.xml validates