Cron validation regex generator (based on http://stackoverflow.com/questions/2362985/verifying-a-cron-expression-is-valid-in-java)

cron-validate-regex.js

function createCronRegex() {
    var regexByField = {};
    regexByField["sec"] = "[0-5]?\\\\d";
    regexByField["min"] = "[0-5]?\\\\d";
    regexByField["hour"] = "[01]?\\\\d|2[0-3]";
    regexByField["day"] = "0?[1-9]|[12]\\\\d|3[01]";
    regexByField["month"] = "[1-9]|1[012]";
    regexByField["dayOfWeek"] = "[0-7]";
    regexByField["year"] = "|\\\\d{4}";

    ["sec", "min", "hour", "day", "month", "dayOfWeek", "year"].forEach(function(field) {
        var number = regexByField[field];
        var range =
            "(?:" + number + ")" +
            "(?:" +
            "(?:-|/|," + ("dayOfWeek" === field ? "|#" : "") + ")" +
            "(?:" + number + ")" +
            ")?";
        if (field === "dayOfWeek") range += "(?:L)?";
        if (field === "month") range += "(?:L|W)?";
        regexByField[field] = "\\\\?|\\\\*|" + range + "(?:," + range + ")*";
    });

    var monthValues = "JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC";
    var monthRange = "(?:" + monthValues + ")(?:(?:-)(?:" + monthValues + "))?";
    regexByField["month"] += "|\\\\?|\\\\*|" + monthRange + "(?:," + monthRange + ")*";

    var dayOfWeekValues = "MON|TUE|WED|THU|FRI|SAT|SUN";
    var dayOfWeekRange = "(?:" + dayOfWeekValues + ")(?:(?:-)(?:" + dayOfWeekValues + "))?";
    regexByField["dayOfWeek"] += "|\\\\?|\\\\*|" + dayOfWeekRange + "(?:," + dayOfWeekRange + ")*";

    return '^\\\\s*($' +
        '|#' +
        '|\\\\w+\\\\s*=' +
        "|" +
        "(" + regexByField["sec"] + ")\\\\s+" +
        "(" + regexByField["min"] + ")\\\\s+" +
        "(" + regexByField["hour"] + ")\\\\s+" +
        "(" + regexByField["day"] + ")\\\\s+" +
        "(" + regexByField["month"] + ")\\\\s+" +
        "(" + regexByField["dayOfWeek"] + ")(|\\\\s)+" +
        "(" + regexByField["year"] + ")" +
        ")$";
}

CronRegex.java

package regex;

import org.junit.Test;

import java.util.HashMap;
import java.util.List;
import java.util.Map;

import static regex.CronRegex.Field.*;
import static java.util.Arrays.asList;
import static org.hamcrest.CoreMatchers.equalTo;
import static org.junit.Assert.assertThat;
import static org.junit.Assert.assertTrue;

public class CronRegex {

    @Test public void regression() {
        String regex = "^\\s*($|#|\\w+\\s*=|(\\?|\\*|(?:[0-5]?\\d)(?:(?:-|/|,)(?:[0-5]?\\d))?(?:,(?:[0-5]?\\d)(?:(?:-|/|,)(?:[0-5]?\\d))?)*)\\s+(\\?|\\*|(?:[0-5]?\\d)(?:(?:-|/|,)(?:[0-5]?\\d))?(?:,(?:[0-5]?\\d)(?:(?:-|/|,)(?:[0-5]?\\d))?)*)\\s+(\\?|\\*|(?:[01]?\\d|2[0-3])(?:(?:-|/|,)(?:[01]?\\d|2[0-3]))?(?:,(?:[01]?\\d|2[0-3])(?:(?:-|/|,)(?:[01]?\\d|2[0-3]))?)*)\\s+(\\?|\\*|(?:0?[1-9]|[12]\\d|3[01])(?:(?:-|/|,)(?:0?[1-9]|[12]\\d|3[01]))?(?:,(?:0?[1-9]|[12]\\d|3[01])(?:(?:-|/|,)(?:0?[1-9]|[12]\\d|3[01]))?)*)\\s+(\\?|\\*|(?:[1-9]|1[012])(?:(?:-|/|,)(?:[1-9]|1[012]))?(?:L|W)?(?:,(?:[1-9]|1[012])(?:(?:-|/|,)(?:[1-9]|1[012]))?(?:L|W)?)*|\\?|\\*|(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)(?:(?:-)(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC))?(?:,(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)(?:(?:-)(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC))?)*)\\s+(\\?|\\*|(?:[0-6])(?:(?:-|/|,|#)(?:[0-6]))?(?:L)?(?:,(?:[0-6])(?:(?:-|/|,|#)(?:[0-6]))?(?:L)?)*|\\?|\\*|(?:MON|TUE|WED|THU|FRI|SAT|SUN)(?:(?:-)(?:MON|TUE|WED|THU|FRI|SAT|SUN))?(?:,(?:MON|TUE|WED|THU|FRI|SAT|SUN)(?:(?:-)(?:MON|TUE|WED|THU|FRI|SAT|SUN))?)*)(|\\s)+(\\?|\\*|(?:|\\d{4})(?:(?:-|/|,)(?:|\\d{4}))?(?:,(?:|\\d{4})(?:(?:-|/|,)(?:|\\d{4}))?)*))$";
        System.out.println(regex);

        String cronRegex = createCronRegex();
        assertThat(cronRegex, equalTo(regex));
    }

    @Test public void quartzExamples() {
        String cronRegex = createCronRegex();

        // example from http://www.quartz-scheduler.org/documentation/quartz-1.x/tutorials/crontrigger
        List<String> quartzExamples = asList(
                "0 0 12 * * ?",
                "0 15 10 ? * *",
                "0 15 10 * * ?",
                "0 15 10 * * ? *",
                "0 15 10 * * ? 2005",
                "0 * 14 * * ?",
                "0 0/5 14 * * ?",
                "0 0/5 14,18 * * ?",
                "0 0-5 14 * * ?",
                "0 10,44 14 ? 3 WED",
                "0 15 10 ? * MON-FRI",
                "0 15 10 15 * ?",
                // "0 15 10 L * ?", broken
                "0 15 10 ? * 6L",
                "0 15 10 ? * 6L 2002-2005",
                "0 15 10 ? * 6#3",
                "0 0 12 1/5 * ?",
                "0 11 11 11 11 ?"
        );
        for (String quartzExample : quartzExamples) {
            assertTrue(quartzExample, quartzExample.matches(cronRegex));
        }
    }

    public enum Field {
        sec, min, hour, day, month, dayOfWeek, year
    }

    // originally copied from http://stackoverflow.com/questions/2362985/verifying-a-cron-expression-is-valid-in-java
    public static String createCronRegex() {
        Map<Field, String> regexByField = new HashMap<Field, String>();

        regexByField.put(sec, "[0-5]?\\d");
        regexByField.put(min, "[0-5]?\\d");
        regexByField.put(hour, "[01]?\\d|2[0-3]");
        regexByField.put(day, "0?[1-9]|[12]\\d|3[01]");
        regexByField.put(month, "[1-9]|1[012]");
        regexByField.put(dayOfWeek, "[0-6]");
        regexByField.put(year, "|\\d{4}");

        // expand regex to contain different time specifiers
        for (Field field : Field.values()) {
            String number = regexByField.get(field);
            String range =
                    "(?:" + number + ")" +
                    "(?:" +
                        "(?:-|/|," + (dayOfWeek == field ? "|#" : "") + ")" +
                        "(?:" + number + ")" +
                    ")?";
            if (field == dayOfWeek) range += "(?:L)?";
            if (field == month) range += "(?:L|W)?";
            regexByField.put(field, "\\?|\\*|" + range + "(?:," + range + ")*");
        }

        // add string specifiers
        String monthValues = "JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC";
        String monthRange = "(?:" + monthValues + ")(?:(?:-)(?:" + monthValues + "))?";
        regexByField.put(month, regexByField.get(month) + "|\\?|\\*|" + monthRange + "(?:," + monthRange + ")*");

        String dayOfWeekValues = "MON|TUE|WED|THU|FRI|SAT|SUN";
        String dayOfWeekRange = "(?:" + dayOfWeekValues + ")(?:(?:-)(?:" + dayOfWeekValues + "))?";
        regexByField.put(dayOfWeek, regexByField.get(dayOfWeek) + "|\\?|\\*|" + dayOfWeekRange + "(?:," + dayOfWeekRange + ")*");

        return "^\\s*($" +
                "|#" +
                "|\\w+\\s*=" +
                "|" +
                "(" + regexByField.get(sec) + ")\\s+" +
                "(" + regexByField.get(min) + ")\\s+" +
                "(" + regexByField.get(hour) + ")\\s+" +
                "(" + regexByField.get(day) + ")\\s+" +
                "(" + regexByField.get(month) + ")\\s+" +
                "(" + regexByField.get(dayOfWeek) + ")(|\\s)+" +
                "(" + regexByField.get(year) + ")" +
                ")$";
    }
}

I think the 4 ''-s are unnecessary in the JS code, you only need two of each.