java convert regular expression into string

It is easy to check a string with regular expression, but there is no direct method to generate string from regular expression. I hardly need it but found nothing. So I myself generated the following program to generate string using regular expression and check back the string with given regular expression. The program i generated will sufficient for normal expression. It will cover my back. It will not parse high level expression.

Output example using random regex generator

Regex: [a-z][a-z0-9]{8,10}@[a-z0-9]{5,9}.com
Matches: [a-z][a-z0-9]{8,10}@[a-z0-9]{5,9}.com, String: bak65qvkpp@3qosue.com, Length: 21



Regex: [\w][\D]{0,10}[\d\W]{1,10}[abc.][abc09]{1,10}
Matches: [\w][\D]*[\d\W]+[abc.][abc09]+, String: aLchJL,e’‘.c9bca, Length: 16



Regex: [A-Z]{4}-#%[\d]{3}
Matches: [A-Z]{4}-#%[\d]{3}, String: BAKZ-#%114, Length: 10



Regex: www.[a-z][a-z0-9-_]{5,15}[a-z].com
Matches: www.[a-z][a-z0-9-_]{5,15}[a-z].com, String: www.jj829x10cq9ngilk.com, Length: 24



Regex: 01[7856][0-9]{2}-[0-9]{6}
Matches: 01[7856][0-9]{2}-[0-9]{6}, String: 01700-736202, Length: 12

StringRegex.java (main class)

import java.util.Random;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 *
 * @author Pritom K Mondal
 * @version 1.0
 */
public class StringRegex {
    public static void main(String[] args) {
        String regex = "[a-z][a-z0-9]{8,10}@[a-z0-9]{5,9}.com";
        StringRegex.stringFromRegex(regex);
        regex = "[\\w][\\D]*[\\d\\W]+[abc.][abc09]+";
        StringRegex.stringFromRegex(regex);
        regex = "[A-Z]{4}-#%[\\d]{3}";
        StringRegex.stringFromRegex(regex);
        regex = "www.[a-z][a-z0-9-_]{5,15}[a-z].com";
        StringRegex.stringFromRegex(regex);
        regex = "01[7856][0-9]{2}-[0-9]{6}";
        StringRegex.stringFromRegex(regex);
    }
    
    public static void stringFromRegex(String regex) {         
        String regexConverted = cvtLineTerminators(regex);
        System.out.println("Regex: " + regexConverted);
        String fullString = "";
        fullString = parse(regexConverted, "");
        if(fullString.matches(regex)) {
            System.out.println("Matches: " + regex + ", String: " + fullString + ", Length: " + fullString.length());
        } else {
            System.err.println("Matches Failed: " + regex + ", String: " + fullString + ", :Length: " + fullString.length());
        }
        System.out.println("\n\n");
    }
    
    private static String parse(String regex, String fullString) {
        Random random = new Random();
        if(regex.trim().length() > 0) {
            Boolean allow = false, processed = false;
            if(regex.startsWith("[") 
                    && (regex.substring(0, regex.indexOf("]") + 1).matches("(.*)[a-z]\\-[a-z](.*)") 
                    || regex.substring(0, regex.indexOf("]") + 1).matches("(.*)[A-Z]\\-[A-Z](.*)") 
                    || regex.substring(0, regex.indexOf("]") + 1).matches("(.*)d(.*)") 
                    || regex.substring(0, regex.indexOf("]") + 1).matches("(.*)D(.*)") 
                    || regex.substring(0, regex.indexOf("]") + 1).matches("(.*)w(.*)") 
                    || regex.substring(0, regex.indexOf("]") + 1).matches("(.*)W(.*)") 
                    || regex.substring(0, regex.indexOf("]") + 1).matches("\\[([a-z]*)([0-9]*).\\]") 
                    || regex.substring(0, regex.indexOf("]") + 1).matches("\\[[A-Z].\\]") 
                    || regex.substring(0, regex.indexOf("]") + 1).matches("(.*)[0-9]\\-[0-9](.*)"))) {
                allow = true;
            }
            if(allow) {
                int start = regex.indexOf("[");
                int end = regex.indexOf("]", start);
                String part = regex.substring(start, end + 1);
                regex = regex.substring(end + 1);
                int pos1 = 1, pos2 = 1;
                if(regex.startsWith("{")) {
                    start = 0;
                    String pos = "";
                    end = regex.indexOf("}", start);
                    pos = regex.substring(start + 1, end);
                    //part = part + regex.substring(start, end + 1);
                    regex = regex.substring(end + 1);
                    
                    if(pos.trim().length() > 0) {
                        if(pos.contains(",")) {
                            String[] poss = pos.split(",");
                            pos1 = Integer.parseInt(poss[0]);
                            pos2 = Integer.parseInt(poss[1]);
                        } else {
                            pos1 = Integer.parseInt(pos);
                            pos2 = Integer.parseInt(pos);
                        }
                    }
                }
                //System.out.println("Pos: " + pos1 + " to " + pos2);
                
                StringBuilder sb = new StringBuilder();                
                String str = "";
                String printInfo = "Processing-1: " + part;
                int tried = 0;
                while(sb.toString().length() == 0 && tried <= 5) {
                    tried++;
                    str = RandomString.nextString(2000);
                    try {
                        Matcher m = Pattern.compile(part).matcher(str);
                        while (m.find()) {
                            sb.append(m.group(0).toString());
                        }
                    } catch (Exception ex) {
                        System.out.println("Exception: " + ex.getMessage());
                    }
                }
                if(sb.toString().length() > 0) {
                    processed = true;
                    str = sb.toString();
                    while(true) {
                        if(str.length() < pos2) {
                            str = str.concat(str);
                        } else {
                            break;
                        }
                    }
                    printInfo += " = (" + pos1 + ", " + pos2 + "): ";
                    String cutString = str.substring(0, pos1);
                    if(pos2 - pos1 > 0) {
                        pos2 = random.nextInt(pos2 - pos1);
                        cutString += str.substring(pos1, pos2 + pos1);
                    }
                    printInfo += cutString;
                    //System.out.println(printInfo);
                    fullString += cutString;
                } else {
                    regex = part + regex;
                }
            } 
            if(regex.startsWith("[") && !processed) {
                processed = true;
                int start = regex.indexOf("[");
                int end = regex.indexOf("]", start);
                String part = regex.substring(start + 1, end);
                regex = regex.substring(end + 1);
                String printInfo = "Processing-2: " + part;
                int pos1 = 1, pos2 = 1;
                if(regex.startsWith("{")) {
                    start = 0;
                    String pos = "";
                    end = regex.indexOf("}", start);
                    pos = regex.substring(start + 1, end);
                    regex = regex.substring(end + 1);
                    
                    if(pos.trim().length() > 0) {
                        if(pos.contains(",")) {
                            String[] poss = pos.split(",");
                            pos1 = Integer.parseInt(poss[0]);
                            pos2 = Integer.parseInt(poss[1]);
                        } else {
                            pos1 = Integer.parseInt(pos);
                            pos2 = Integer.parseInt(pos);
                        }
                    }
                }
                String pushString = "";
                for(int i = 0; i < pos1; i++) {
                    pushString += part;
                }
                if(pos2 - pos1 > 0) {
                    pos2 = random.nextInt(pos2 - pos1);
                    for(int i = 0; i < pos2; i++) {
                        pushString += part;
                    }
                }
                printInfo += "{" + pos1 + "," + pos2 + "} = " + pushString;
                //System.out.println(printInfo);
                fullString += pushString;
            } 
            if(!processed) {
                //System.out.println("Handled: " + regex);
                regex = "[" + regex;
                int fa = regex.indexOf("]"), f2 = regex.indexOf("{"), f3 = regex.indexOf("[", 1);
                if(fa > f3) fa = f3;
                if(fa > f2) fa = f2;
                if(fa < 0) {
                    if(f2 < f3 && f2 >= 0) {
                        fa = f2;
                    } else if(f3 > f2 && f3 >= 0) {
                        fa = f3;
                    } else {
                        fa = regex.length();
                    }
                }
                //regex = regex.substring(0, fa) + "]" + regex.substring(fa);
                regex = regex.substring(0, 2) + "]" + regex.substring(2);
                //System.out.println("Handled: " + regex);
            }
            fullString = parse(regex, fullString);
        }
        return fullString;
    }
    
    private static String cvtLineTerminators (String s) {
        s = s.replaceAll("\\*", "{0,10}");
        s = s.replaceAll("\\+", "{1,10}");
        
        StringBuffer sb = new StringBuffer ();
        int oldindex = 0, newindex;
        while ((newindex = s.indexOf ("\\n", oldindex)) != -1) {
            sb.append (s.substring (oldindex, newindex));
            oldindex = newindex + 2;
            sb.append ('\n');
        }
        sb.append (s.substring (oldindex));

        s = sb.toString ();
        sb = new StringBuffer ();
        oldindex = 0;
        while ((newindex = s.indexOf ("\\r", oldindex)) != -1) {
            sb.append (s.substring (oldindex, newindex));
            oldindex = newindex + 2;
            sb.append ('\r');
        }
        sb.append (s.substring (oldindex));

        s = sb.toString ();
        sb = new StringBuffer ();
        oldindex = 0;
        while ((newindex = s.indexOf ("\\s", oldindex)) != -1) {
            sb.append (s.substring (oldindex, newindex));
            oldindex = newindex + 2;
            sb.append (" ");
        }
        sb.append (s.substring (oldindex));
        
        return sb.toString();
   }
}

RandomString.java

import java.util.Random;

/**
 *
 * @author Pritom K Mondal
 */
public class RandomString {
    private static final char[] symbols = new char[10 + 26 + 26 + 3 + 8];
    private static final char[] chars = new char[52];
    private static final char[] numbers = new char[10];
    
    static {
        for (int idx = 0; idx < 10; ++idx) {
            numbers[idx] = (char) ('0' + idx);
        }
        for (int idx = 10; idx < 36; ++idx) {
            chars[idx - 10] = (char) ('a' + idx - 10);
        }
        for (int idx = 36; idx < 62; ++idx) {
            chars[idx - 10] = (char) ('A' + idx - 36);
        }
        /**
         * String
         */
        int total = 0;
        for(int idx = 48; idx <= 57; idx++, total++) {
            symbols[total] = (char) idx;
        }// 10 (10)
        for(int idx = 65; idx <= 90; idx++, total++) {
            symbols[total] = (char) idx;
        } // 26 (36)
        for(int idx = 97; idx <= 122; idx++, total++) {
            symbols[total] = (char) idx;
        } // 26 (62)
        for(int idx = 44; idx <= 46; idx++, total++) {
            symbols[total] = (char) idx;
        } // 3 (65)
        int[] copyFrom = { 64, 95, 44, 46, 45, 145, 146, 35 };
                         //@   _   ,   .   -   ‘     ’    #
        for(int idx = 0; idx < copyFrom.length; idx++, total++) {
            symbols[total] = (char) copyFrom[idx];
        } // 8 (73)
        //System.out.println(total);
    }
    
    private static Random random = new Random();
    private static char[] buf;
    
    public static String fullString() {
        return new String(symbols);
    }
    
    public static String nextString(int length) {
        if (length < 1) {
            throw new IllegalArgumentException("length < 1: " + length);
        }
        buf = new char[length];
        //System.out.println(symbols);
        for (int idx = 0; idx < buf.length; ++idx) {
            buf[idx] = symbols[random.nextInt(symbols.length)];
        }
        return new String(buf);
    }
    
    public static String nextNumber(int length) {
        if (length < 1) {
            throw new IllegalArgumentException("length < 1: " + length);
        }
        buf = new char[length];
        for (int idx = 0; idx < buf.length; ++idx) {
            buf[idx] = numbers[random.nextInt(numbers.length)];
        }
        return new String(buf);
    }
    
    public static String nextCharacter(int length) {
        if (length < 1) {
            throw new IllegalArgumentException("length < 1: " + length);
        }
        buf = new char[length];
        for (int idx = 0; idx < buf.length; ++idx) {
            buf[idx] = chars[random.nextInt(chars.length)];
        }
        return new String(buf);
    }
}