This example shows what can be done with a bit more time to develop a script. This example includes use of regular expressions to extract values from the email, and searching with a JQL query to find an existing Jira issue. It results in a behaviour similar to the Regexp Field Processor.

// For more detail and background information regarding this Script please see the following Documentation: https://thepluginpeople.atlassian.net/l/cp/CZKAm2Vw
// Create Regexp pattern to extract external id
var pattern = Java.type("java.util.regex.Pattern").compile("Case\\s#:\\s([0-9]+)");
var matcher = pattern.matcher(body);
var foundExternalId = "";

// Set the value of the Custom Field which will hold the External ID value on the Issue
var externalIDField = "customfield_10122";
 
// If able to extract issue key, set variable
if (matcher.find()) {
    var group = matcher.group(1);
    if (group) {
        foundExternalId = group;
    }
}
 
var issueWithExternalId = "";
if (foundExternalId) {
    print("external ID extracted: "+foundExternalId);
    
    // Use Thread Utils to find a list of Issues which hold the 'foundExternalId' in the 'externalIDField'. This method will 
    // apply Thread Match conditions defined in the Profile (only match open issues, matches JQL, etc.). 
    // Additionally, the returned list will be ordered by Creation Date (Ascending) therefore the Issue at index 0 would be the oldest valid Issue
    var results = jemhThreadUtils.getFilteredAssociatedIssuesFromExternalId(externalIDField, foundExternalId);
      
    // Get first issue if exists and add via issueKey directive (Could be empty)
    if (results.size()>0) {
        var issue = results[0];
        issueWithExternalId = issue.getKey();
        print("Found issue using External ID match:"+issueWithExternalId);
    } else {
        print("No issues match found using External ID match");
    }
}
 
if (!issueWithExternalId.isEmpty()) {
        resultMap.put("issueKey",issueWithExternalId)
} else if (!foundExternalId.isEmpty()) {
        resultMap.put(externalIDField, foundExternalId);
}

var filter = PagerFilter.static.getUnlimitedFilter();
var builder = JqlQueryBuilder.static.newBuilder();

var filter = Java.type("com.atlassian.jira.web.bean.PagerFilter").getUnlimitedFilter();
var builder = Java.type("com.atlassian.jira.jql.builder.JqlQueryBuilder").newBuilder();

Due to issues highlighted on the following page,https://thepluginpeople.atlassian.net/wiki/spaces/JEMH/pages/3974856709/Using+Java+17#Classnotfound-exception-when-using-%E2%80%9Cjava.type%E2%80%9D. It means that if you are using Java 17 with Nashorn installed within jira-install/lib then you will need to use JEMH 4.1.33+ and then use the following instead of lines 20 and 21:

• var filter = pagerFilter; var builder = jqlQueryBuilder;

//create regexp pattern to extract external id
var pattern = Java.type("java.util.regex.Pattern").compile("Case\\s#:\\s([0-9]+)");
var matcher = pattern.matcher(body);
var foundExternalId = "";
 
//if able to extract issue key, set variable
if (matcher.find()) {
    var group = matcher.group(1);
    if (group) {
        foundExternalId = group;
    }
}
 
var issueWithExternalId = "";
if (foundExternalId) {
    print("external ID extracted: "+foundExternalId);
    //get the user to run the JQL query as
    var user = userManager.getUserByName("admin");
      
    var filter = Java.type("com.atlassian.jira.web.bean.PagerFilter").getUnlimitedFilter();
    var builder = Java.type("com.atlassian.jira.jql.builder.JqlQueryBuilder").newBuilder();
      
    //construct the JQL query
    var query = builder.where().field("External ID").like(foundExternalId).buildQuery();
      
    //get list of issues (could be empty)
    var results = searchService.search(user, query, filter).getIssues();
      
    //get first issue if exists and add via issueKey directive
    if (results.size()>0) {
        var issue = results[0];
        issueWithExternalId = issue.getKey();
        print("Found issue using JQL "+query+" : "+issueWithExternalId);
    } else {
        print("No issues match using JQL: "+query);
    }
}
 
if (!issueWithExternalId.isEmpty()) {
        resultMap.put("issueKey",issueWithExternalId)
} else if (!foundExternalId.isEmpty()) {
        resultMap.put("External ID",foundExternalId);
}

//create regexp pattern to extract external id
var pattern = Java.type("java.util.regex.Pattern").compile("Case\\s#:\\s([0-9]+)");
var matcher = pattern.matcher(body);
var foundExternalId = "";
 
//if able to extract issue key, set variable
if (matcher.find()) {
    var group = matcher.group(1);
    if (group) {
        foundExternalId = group;
    }
}
 
var issueWithExternalId = "";
if (foundExternalId) {
    print("external ID extracted: "+foundExternalId);
    //get the user to run the JQL query as
    var user = userManager.getUserByKey("admin");
      
    var filter = Java.type("com.atlassian.jira.web.bean.PagerFilter").getUnlimitedFilter();
    var builder = Java.type("com.atlassian.jira.jql.builder.JqlQueryBuilder").newBuilder();
      
    //construct the JQL query
    var query = builder.where().field("External ID").like(foundExternalId).buildQuery();
      
    //get list of issues (could be empty)
    var results = searchService.search(user, query, filter).getResults();
      
    //get first issue if exists and add via issueKey directive
    if (results.size()>0) {
        var issue = results[0];
        issueWithExternalId = issue.getKey();
        print("Found issue using JQL "+query+" : "+issueWithExternalId);
    } else {
        print("No issues match using JQL: "+query);
    }
}
 
if (!issueWithExternalId.isEmpty()) {
        resultMap.put("issueKey",issueWithExternalId)
} else if (!foundExternalId.isEmpty()) {
        resultMap.put("External ID",foundExternalId);
}