Parsing version number from a Java source file
Since you are only looking for a single occurrence of the variable in the file you could  match the whole file at once. For example like this:

version = {-&gt;
    def text = file('src/main/org/h2/engine/Constants.java').text
    def extract = {name -&gt;
        nameValue = text.replaceAll(/(?s)^.*public\s+static\s+final\s+int\s+$name\s+=\s*(\d+);.*$/, '$1')
        text.length() == nameValue.length() ? "Cannot find name" : nameValue;
    }
    extract('VERSION_MAJOR') + '.' + extract('VERSION_MINOR') + '.' + extract('BUILD_ID')
}()


Instead of getting the lines this script reads the file to one single String, and then uses a regular expression to replace the while text with the variable of interest. 

In order to do this, we first make sure that the . in the regexp will match newlines by adding (?s) to the pattern. We then match everything from the start (^.*) up until public static final int $name, capture the value with (\d+), and then match everything to the end of the file with .*$.

Although the regular expression is slightly more involved, this way makes it easier to check whether the replacement has worked and prevents a NullPointerException. We can now simply compare the length of the original text with the length of the result. If they are the same the replacement has not succeeded and we can return a custom error-message, otherwise we found the variable of interest.