This is a java library that, given a regex pattern, allows to:

1. Generate matching strings
2. Iterate through unique matching strings
3. Generate not matching strings

Status
Try it now
Usage
Supported Syntax
Configuration
Limitations
Other similar libraries
Support

Build status:

Latest Release	Latest snapshot

Follow the link to Online IDE with created project: JDoodle. Enter your pattern and see the results.

<dependency>
    <groupId>com.github.curious-odd-man</groupId>
    <artifactId>rgxgen</artifactId>
    <version>1.4</version>
</dependency>

<project>
    <repositories>
        <repository>
            <id>snapshots-repository</id>
            <url>https://oss.sonatype.org/content/repositories/snapshots/</url>
        </repository>
    </repositories>
    
    <!--  .... -->
    
    <dependency>
        <groupId>com.github.curious-odd-man</groupId>
        <artifactId>rgxgen</artifactId>
        <version>1.5-SNAPSHOT</version>
    </dependency>
</project>

Changes in snapshot:

None at the moment.

public class Main {
    public static void main(String[] args){
        RgxGen rgxGen = new RgxGen("[^0-9]*[12]?[0-9]{1,2}[^0-9]*");         // Create generator
        String s = rgxGen.generate();                                        // Generate new random value
        Optional<BigInteger> estimation = rgxGen.getUniqueEstimation();      // The estimation (not accurate, see Limitations) how much unique values can be generated with that pattern.
        StringIterator uniqueStrings = rgxGen.iterateUnique();               // Iterate over unique values (not accurate, see Limitations)
        String notMatching = rgxGen.generateNotMatching();                   // Generate not matching string
    }
}

public class Main {
    public static void main(String[] args){
        RgxGen rgxGen = new RgxGen("[^0-9]*[12]?[0-9]{1,2}[^0-9]*");         // Create generator
        Random rnd = new Random(1234);
        String s = rgxGen.generate(rnd);                                     // Generate first value
        String s1 = rgxGen.generate(rnd);                                    // Generate second value
        String s2 = rgxGen.generate(rnd);                                    // Generate third value
        String notMatching = rgxGen.generateNotMatching(rnd);                // Generate not matching string
        // On each launch s, s1 and s2 will be the same
    }
}

RgxGen can be configured on global or instance level.

Please refer to the following enum for all available properties: com.github.curiousoddman.rgxgen.config.RgxGenOption.

Each property value will be looked up in this order:

1. Local RgxGen instance config
2. Global RgxGen config
3. Default values

Use new RgxGenProperties() to create properties object. RgxGenProperties extends java.util.Properties and can be used in all the same ways.

public class Main {
    public static void main(String[] args){
        // Create properties object (RgxGenProperties extends java.util.Properties)
        RgxGenProperties properties = new RgxGenProperties();
        // Set value "20" for INFINITE_PATTERN_REPETITION option in properties
        RgxGenOption.INFINITE_PATTERN_REPETITION.setInProperties(properties, 20);
        // ... now properties can be passed to RgxGen
    }
}

Set a global configuration using RgxGen.setDefaultProperties(properties);

public class Main {
    public static void main(String[] args){
        RgxGenProperties properties = createAndConfigureProperitesObject();
    
        RgxGen rgxGen_1 = new RgxGen("xxx");        // Created for example purposes
        // Set default properties. 
        // NOTE! only instances created after setDefaultProperties are affected.
        // e.g. rgxGen_1 will have default value of INFINITE_PATTERN_REPETITION option
        // and rgxGen_2 will have value "20" for the property, unless local config specified.
        RgxGen.setDefaultProperties(properties);
        RgxGen rgxGen_2 = new RgxGen("xxx");
    }
}

Set a local configuration using rgxGen.setProperties(localProperties); on existing RgxGen instance.

public class Main {
    public static void main(String[] args){
        RgxGenProperties properties = createAndConfigureProperitesObject();
        RgxGen.setDefaultProperties(properties);
    
        // Create properties object (RgxGenProperties extends java.util.Properties)
        RgxGenProperties localProperties = createAndConfigureLocalProperitesObject();
        RgxGen rgxGen_3 = new RgxGen("xxx"); 
        // Set local configuration for rgxGen_3
        // Note, for options that are not defined in localProperties, will try find option inside properties, since these are set globally prior creation of rgxGen_3 instance creation 
        rgxGen_3.setProperties(localProperties);
    }
}

Currently, these two have very limited support. Please refer to #63. I'm currently working on the solution, but I cannot say when I come up with something.

rgxGen.getUniqueEstimation() - might not be accurate, because it does not count actual unique values, but only counts different states of each building block of the expression. For example: "(a{0,2}|b{0,2})" will be estimated as 6, though actual number of unique values is 5. That is because left and right alternative can produce same value. At the same time "(|(a{1,2}|b{1,2}))" will be correctly estimated to 5, though it will generate same values.

For the similar reasons as with estimations - requested unique values iterator can contain duplicates.

By design a+, a* and a{n,} patterns in regex imply infinite number of characters should be matched. When generating data, that would mean values of infinite length might be generated. It is highly doubtful anyone would require a string of infinite length, thus I've artificially limited repetitions in such patterns to 100 symbols, when generating random values. This value can be changed - please refer to configuration section.

On the contrast, when generating unique values - the number of maximum repetitions is Integer.MAX_VALUE.

Use a{n,m} if you require some specific number of repetitions. It is suggested to avoid using such infinite patterns to generate data based on regex.

The general rule is - I am trying to generate not matching strings of same length as would be matching strings, though it is not always possible. For example pattern . - any symbol - would yield empty string as not matching string. Another example a{0,2} - this pattern could yield empty string, but for not matching string the resulting strings would be only 1 or 2 symbols long. I chose these approaches because they seem predictable and easier to implement.

There are 2 more libraries available to achieve same goal:

1. https://github.com/mifmif/Generex
2. http://code.google.com/p/xeger

Though I found they have following issues:

1. All of them build graph which can easily produce OOM exception. For example pattern a{60000}, or IPV6 regex pattern.
2. Alternatives - only 2 alternatives gives equal probability of each alternative to appear in generated values. For example: (a|b) the probability of a and b is equal. For (a|b|c) it would be expected to have a or b or c with probability 33.(3)% each. Though really the probabilities are a=50%, and b=25% and c=25% each. For longer alternatives you might never get the last alternative.
3. They are quite slow
4. Lightweight. This library does not have any dependencies.

I plan to support this library, so you're welcome to open issues or reach me by e-mail in case of any questions. Any suggestions, feature requests or bug reports are welcome!

Please vote up my answer on StackOverflow to help others find this library.