Since ModSecurity is based on regular expressions. Writing rules requires developing and testing such expressions. The following tools can help you in analyzing and testing them:
- The Regex Coach is simple and powerful. You simply type your expression in the upper box and the text to match in the lower box. Any matches, if found, are highlighted in the text. In between the boxes you can control the regular expression flags such as "ignore case" or "global match". The Regex Coach does not stop there, it provides insight into the regular expression matching process by showing a tree view of the regular expression and letting you follow the matching process, step by step. A weak point is that sometimes the Regex Coach has difficulty in handling the regular expressions we use in the ModSecurity Enhanced Rule Set.
- Expresso - An alternative tool that works well is Expresso. While free, it is not typical of open source software. It uses the .NET framework and you may be required to pay for certain other components. It has good peripheral features such as a library of regular expressions and the ability to save your test work in a project file, but is more complex than the Regex Coach and seems to have less information about the internals of regular expressions.