Who has the algorithm of translating RegExp into "natural language"?

Question

I want to "translate" a regular expression into a human-readable explanation.

For example: /^[a-z0-9_]{6,18}/ will be translated to something like:

A string start with 6 to 18 char(s) in a range of a to z, 0 to 9 and _.

Answer 1

Expresso, a free tool, does this (and does a good job too).

Expresso 3.0 on Microsoft's regular expression Webcast series

Answer 2

Using RegexBuddy (Paid however in my opinion the best tool available) the expression is documented as below:

// ^[a-z0-9_]{6,18}
// 
// Options: ^ and $ match at line breaks
// 
// Assert position at the beginning of a line (at beginning of the string or after a line break character) «^»
// Match a single character present in the list below «[a-z0-9_]{6,18}»
//    Between 6 and 18 times, as many times as possible, giving back as needed (greedy) «{6,18}»
//    A character in the range between “a” and “z” «a-z»
//    A character in the range between “0” and “9” «0-9»
//    The character “_” «_»

The tool also allows you to select a line in the documentation and showing you the part in the regular expression.