这将适用于所提供的示例。

def majorMinorPatchBuild = '1.20.30.44'
def majorMinorPatch
if (majorMinorPatchBuild.toList().count('.') < 3) {
    majorMinorPatch = majorMinorPatchBuild
} else {
    majorMinorPatch = majorMinorPatchBuild.split('\\.')[0..-2].join('.')
}
assert majorMinorPatch == '1.20.30'

请注意，如果输入数据有一些格式偏差，则必须实现一些错误处理。
测试更多数据：

[
        '1.2.3',
        '1.2.3.4',
        '11.22.33.44',
        '111.222.333.444'
].each {
    def majorMinorPatchBuild = it
    def majorMinorPatch
    if (majorMinorPatchBuild.toList().count('.') < 3) {
        majorMinorPatch = majorMinorPatchBuild
    } else {
        majorMinorPatch = majorMinorPatchBuild.split('\\.')[0..-2].join('.')
    }

    println("major.minor.patch: '$it' -> '$majorMinorPatch'")
}

输出量：

major.minor.patch: '1.2.3' -> '1.2.3'
major.minor.patch: '1.2.3.4' -> '1.2.3'
major.minor.patch: '11.22.33.44' -> '11.22.33'
major.minor.patch: '111.222.333.444' -> '111.222.333'

在Groovy中，这基本上应该是一行程序。

String listBased = '1.20.30.44'.split( /\./ ).take 3 join '.'
String regexBased = '1.20.30.44'.replaceFirst( /(.+)\.[^.]+/, '$1' )

assert listBased == '1.20.30'
assert regexBased == '1.20.30'

此表达式返回versionString中最后一个点之前的子字符串，因此结果与您在问题中指定的结果完全相同：

versionString = '1.20.30.44'
println versionString[0..versionString.lastIndexOf('.')-1]
// => 1.20.30