This script should work with scriptrunner in JIRA to a) get an issue; b) match some stuff in the description; and c) get values from custom fields.

get-jira-issue-matchstuff.groovy

//Import classes you need
import com.atlassian.jira.ComponentAccessor;
import com.atlassian.jira.issue.Issue;
import com.atlassian.jira.issue.CustomFieldManager;
import com.atlassian.jira.issue.MutableIssue;
import com.atlassian.jira.issue.IssueManager;
import com.atlassian.jira.component.ComponentAccessor;
import com.atlassian.jira.user.ApplicationUser;
import com.atlassian.jira.security.JiraAuthenticationContext;
import com.atlassian.jira.issue.fields.CustomField;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

// Get some additional information you typically need like current user
JiraAuthenticationContext userMgr = ComponentAccessor.getJiraAuthenticationContext();
ApplicationUser currentUserObj = userMgr.getLoggedInUser();

//Get an IssueManager. you always need one of those.
IssueManager issueMgr = ComponentAccessor.getIssueManager();
CustomFieldManager fieldmgr = ComponentAccessor.getCustomFieldManager();
MutableIssue issue = issueMgr.getIssueObject("JRA-132")  // this is just an example key

//Create the regex matcher(s) you need
def matcher = issue.description =~ /INC\d\d\d\d\d/
try {
    String res = matcher[0]
    log.error "Component matcher found " + res;
} catch ( IndexOutOfBoundsException e ) {
    log.error e;
}

//Print stuff. Just assuming you are playing around/learning in the Scriptrunner console.
log.error "";
log.error "The current user is " + currentUserObj.name;
log.error "The current issue ID is " + issue.id;
log.error "The current issue key is " + issue.key;
log.error "The current issue description is " + issue.description;
log.error "The current issue special field value is " + issue.getCustomFieldValue(fieldmgr.getCustomFieldObject(10603L));

I tried also MutableIssue issue = issueMgr.getIssueByCurrentKey(issue.key) and get error

A little better if I cast Object to String. Still not seeing variable "issue". ScriptRunner is verison 5.5.7

I guess we are back to the original question --

1. how to get an "issue" in the ScriptRunner console w/o hard-coding the issue key?
2. The subsequent code will be used as a post function so the followup to the above question is, after getting the code to work in ScriptRunner, how to modify the code (if modification needed) to run as a post-function?

Is the answer just to use a hard-coded key in the ScriptRunner Console for testing then replace the hard-coded key with "issue.key" when making it a post-function?

Got code to work as post-function by removing entirely the code:

MutableIssue issue = issueMgr.getIssueObject((String) issue.key())

since the "issue" is already defined / exists when it is a post-function. I guess the only way to test is to hard-code the issue key in ScriptRunner.

no luck in my case

CustomFieldManager fieldmgr = ComponentAccessor.getCustomFieldManager()
MutableIssue issue = issueMgr.getIssueObject((String)issue.key())
def customFieldManager = ComponentAccessor.getCustomFieldManager()
def cField = customFieldManager.getCustomFieldObject("customfield_11601")
def cFieldValue = issue.getCustomFieldValue(cField)```