A grab of the current build-thirdparty.xml version description yields many uncertain version strings in terms of comparisons between versions: 2.7.5H3 1.2b3 0.2-cvs 1.0.4.1jboss 3.1beta6 cvs-7-19 2.0.x

 <componentref name="antlr" version="2.7.5H3"/>
 <componentref name="apache-addressing" version="cvsbuild-7-19"/>
 <componentref name="apache-avalon" version="4.1.5"/>
 <componentref name="apache-avalon-logkit" version="1.2"/>
 <componentref name="apache-bcel" version="5.1"/>
 <componentref name="apache-beanutils" version="1.6.0"/>
 <componentref name="apache-bsf" version="2.3.0"/>
 <componentref name="apache-codec" version="1.2.0"/>
 <componentref name="apache-collections" version="2.1"/>
 <componentref name="apache-crimson" version="1.1.1"/>
 <componentref name="apache-digester" version="1.6"/>
 <componentref name="apache-discovery" version="0.2"/>
 <componentref name="apache-fileupload" version="1.0"/>
 <componentref name="apache-httpclient" version="2.0.2"/>
 <componentref name="apache-jaxme" version="0.2-cvs"/>
 <componentref name="apache-lang" version="1.0"/>
 <componentref name="apache-log4j" version="1.2.8"/>
 <componentref name="apache-logging" version="1.0.4.1jboss"/>
 <componentref name="apache-myfaces" version="1.1.0"/>
 <componentref name="apache-pool" version="1.0.1"/>
 <componentref name="apache-scout" version="1.0"/>
 <componentref name="apache-slide" version="1.0.16"/>
 <componentref name="apache-slide" version="1.0.16"/>
 <componentref name="apache-tomcat" version="5.5.12"/>
 <componentref name="apache-velocity" version="1.4jboss"/>
 <componentref name="apache-wss4j" version="cvs-7-19"/>
 <componentref name="apache-xalan" version="j_2.6.0"/>
 <componentref name="apache-xerces" version="2.7.0"/>
 <componentref name="apache-xmlsec" version="1.2"/>
 <componentref name="asm" version="1.5.3"/>
 <componentref name="beanshell" version="1.3.0"/>
 <componentref name="cglib" version="2.1.2jboss"/>
 <componentref name="dom4j" version="1.6.1jboss"/>
 <componentref name="gjt-jpl-util" version="1.0"/>
 <componentref name="gnu-getopt" version="1.0.10"/>
 <componentref name="gnu-regexp" version="1.1.14"/>
 <componentref name="hibernate" version="3.1rc2jboss"/>
 <componentref name="hibernate-annotations" version="3.1beta6"/>
 <componentref name="hibernate-entitymanager" version="3.1beta4"/>
 <componentref name="hsqldb" version="1.8.0.2"/>
 <componentref name="ibm-wsdl4j" version="1.5.1"/>
 <componentref name="jacorb" version="2.2.1jboss"/>
 <componentref name="javassist" version="3.1RC2"/>
 <componentref name="jaxen" version="1.1beta4"/>
 <componentref name="jboss/aop" version="1.3.4"/>
 <componentref name="jboss/cache" version="1.2.4_SP1_FINAL"/>
 <componentref name="jboss/microcontainer" version="1.0.1"/>
 <componentref name="jboss/jbossws" version="snapshot"/>
 <componentref name="jboss/jbossxb" version="snapshot"/>
 <componentref name="jboss/remoting" version="1.4.0_beta"/>
 <componentref name="jboss/serialization" version="1.0.0.RC2"/>
 <componentref name="jfreechart" version="0.9.20"/>
 <componentref name="jgroups" version="2.2.7"/>
 <componentref name="joesnmp" version="0.3.3"/>
 <componentref name="juddi" version="0.9RC4"/>
 <componentref name="junit" version="3.8.1"/>
 <componentref name="junitejb" version="1.4"/>
 <componentref name="objectweb-joramtests" version="1.1"/>
 <componentref name="odmg" version="3.0"/>
 <componentref name="oswego-concurrent" version="1.3.4"/>
 <componentref name="qdox" version="1.4"/>
 <componentref name="retrotranslator" version="0.9.5"/>
 <componentref name="sleepycat" version="1.5.2"/>
 <componentref name="sun-jaf" version="1.0.2"/>
 <componentref name="sun-javacc" version="3.2"/>
 <componentref name="sun-javamail" version="1.3.1"/>
 <componentref name="sun-servlet" version="2.4"/>
 <componentref name="trove" version="2.1.1"/>
 <componentref name="wutka-dtdparser" version="1.2.1"/>
 <componentref name="xdoclet" version="1.2b3"/>
 <componentref name="xml-sax" version="2.0.x"/>

The OSGi R4 version syntax is:

3.2.4 Version
Version specifications are used in several places. A version token has the following grammar:

version ::=
major( '.' minor ( '.' micro ( '.' qualifier )? )? )?
major ::= number // See 1.4.2
minor ::= number
micro ::= number
qualifier ::= ( alphanum | ?_? | '-' )+

A version token must not contain any white space. The default value for a
version is 0.0.0.

3.2.5 Version Ranges
A version range describes a range of versions using a mathematical interval notation. The syntax of a version range is:

version-range ::= interval | atleast
interval ::= ( '[' | '(' ) floor ',' ceiling ( ']' | ')' )
atleast ::= version
floor ::= version
ceiling ::= version

If a version range is specified as a single version, it must be interpreted as the range [version,?). The default for a non-specified version range is 0, which maps to [0.0.0,?).

So the only supported syntax for non-numeric is major.minor.micro.alphanum

To your examples:

3.1.a1 would have to be 3.1.1.a or 3.1.1.alpha

The key thing is coming up with a convention such that within a set of versions there are meaningful comparisons. I'll take a look at the eclipse osgi framework implementation to see how the alphanum is handled to see what convention could be used for comparision.

The version implementation included with the osgi framework classes simply interprets the qualifier element of a version using string compare, and so the qualifier prefix is what has to be consistent to establish a consistent version convention. A final release with an implicit qualifier of 0 is the lowest of any x.y.z.q number as Max said, so whatever the convention alpha, beta, RCn, SPn, etc., the qualifier convention has to result in correct comparision. This also applies to case. "alpha".compareTo("RC1") is in fact gt 0, while "Alpha".compareTo("RC1") lt 0.

package org.osgi.framework;
public class Version implements Comparable {
 private final int major;
 private final int minor;
 private final int micro;
 private final String qualifier;
...
 /**
 * Compares this <code>Version</code> object to another object.
 *
 * <p>
 * A version is considered to be <b>less than </b> another version if its
 * major component is less than the other version's major component, or the
 * major components are equal and its minor component is less than the other
 * version's minor component, or the major and minor components are equal
 * and its micro component is less than the other version's micro component,
 * or the major, minor and micro components are equal and it's qualifier
 * component is less than the other version's qualifier component (using
 * <code>String.compareTo</code>).
 *
 * <p>
 * A version is considered to be <b>equal to</b> another version if the
 * major, minor and micro components are equal and the qualifier component
 * is equal (using <code>String.compareTo</code>).
 *
 * @param object The <code>Version</code> object to be compared.
 * @return A negative integer, zero, or a positive integer if this object is
 * less than, equal to, or greater than the specified
 * <code>Version</code> object.
 * @throws ClassCastException If the specified object is not a
 * <code>Version</code>.
 */
 public int compareTo(Object object) {
 if (object == this) { // quicktest
 return 0;
 }

 Version other = (Version) object;

 int result = major - other.major;
 if (result != 0) {
 return result;
 }

 result = minor - other.minor;
 if (result != 0) {
 return result;
 }

 result = micro - other.micro;
 if (result != 0) {
 return result;
 }

 return qualifier.compareTo(other.qualifier);
 }