Combinations and Permutations. What's the Difference?
In English we use the word "combination" loosely, without thinking if the order of things is important. In other words: So, in Mathematics we use more precise language: In other words: A Permutation is an ordered Combination.
Regular Expressions Reference - Basic Syntax. The regular expressions reference on this website functions both as a reference to all available regex syntax and as a comparison of the features supported by the regular expression flavors discussed in the tutorial.
The reference tables pack an incredible amount of information. To get the most out of them, follow this legend to learn how to read them. The tables have six columns for each regular expression feature. The first four explain the feature. The final two columns indicate whether your two chosen regular expression flavors support this particular feature. Online regular expression testing for Java.