import javax.xml.xpath.XPathFactory
import org.xml.sax.InputSource

def xmlString = '''
<root>
    <OfferItem>
        <OfferItemID>wanted_id_1</OfferItemID>
        <Service>
            <ServiceDetails>
                <PaxJourneyRefID>PJ1</PaxJourneyRefID>
            </ServiceDetails>
        </Service>
        <Service>
            <ServiceDetails>
                <ServiceDetailsRef>
                    <PSRID>S1</PSRID>
                    <ServiceID>SR_YES</ServiceID>
                </ServiceDetailsRef>
            </ServiceDetails>
        </Service>
    </OfferItem>
    <OfferItem>
        <OfferItemID>not_id_1</OfferItemID>
        <Service>
            <ServiceDetails>
                <PaxJourneyRefID>PJ1</PaxJourneyRefID>
            </ServiceDetails>
        </Service>
        <Service>
            <ServiceDetails>
                <ServiceDetailsRef>
                    <PSRID>S1</PSRID>
                    <ServiceID>SR_NO</ServiceID>
                </ServiceDetailsRef>
            </ServiceDetails>
        </Service>
    </OfferItem>
</root>
'''

// Create XPath expression
def xpath = "//OfferItem[Service/ServiceDetails/ServiceDetailsRef/ServiceID='SR_YES']/OfferItemID/text()"

// Create XPath object
def xpathInstance = XPathFactory.newInstance().newXPath()

// Evaluate XPath expression
def offerItemId = xpathInstance.evaluate(xpath, new InputSource(new StringReader(xmlString)))

println "OfferItemID: $offerItemId"
assert offerItemId == 'wanted_id_1'