1. In http://www.rise4fun.com/rex (opens in new tab) write two regexes on separate lines, say R and S
2. Click permalink to create a duel where R is visible and S is hidden.This creates a permanent web link containing the duel.

You have created your own duel!

Example

Enter regexes R and S (note that comments (opens in new tab)can be added to regexes) and click permalink:

This particular example is the link http://www.rise4fun.com/Rex/abG (opens in new tab).When you visit the link, only R is shown, S remains secret:When you now ask rex, the following results are shown:

About .NET regex constructs

Most regex constructs are explained in .NET regexes (opens in new tab).

Using character classes

Regarding character classes, a less known but useful construct is subtraction operation in character classes (opens in new tab):

• [character_class-[character_class]]

Using comments

When writing regexes, you can insert comments (opens in new tab), e.g., to provide a hint in the given regex in the duel, for solving the secret regex

• (?# comment text)

Examples:

• [a-z-[d-g]](?# the set of all lowercase letters except d, e, f and g)note the two different meanings of -, used in this character class
• [w-[d]](?# the set of all word characters except digits)e.g., [w-[d]] is equivalent (in ASCII range) to [a-zA-Z_] or [w-[0-9]]
• [a-[a]](?# the empty set of characters)
• ^[a-z]+$(?# regex matching one or more lowercase letters)
• ^d{2,}$(?# regex matching two or more decimal digits)e.g., ^d{2,}$ is equivalent to ^[0-9]{2,}$ or ^dd+$

Rex is a simple command line tool that takes a .NET regex pattern (opens in new tab)or several regex patterns and generates matching string(s) for them. Rex can also be used through an API. Rex is fast, using a novel approach to solve this problem. When several regexes are provided, rex can be used to generate members that match all the regexes. By default, rex assumes UTF16 encoding of characters, but can also be restricted to ASCII ecoding.

The following example generates 10 strings that start and end with a digit and have at least two characters:

> rex.exe "^d.*d$" /k:10

In order to produce members that contain only ascii characters the /e option can be used. For general instructions regarding usage run:

> rex.exe /?

1) A regex pattern (or several patterns) is translated into a symbolic finite automaton

^[0-9]+|[a-z]$   ==> 

2) The automaton is given to a constraint solver that generates members from it.

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