regex101 is an online tool for building, testing, and debugging regular expressions with support for multiple programming language flavors.

PCRE is the best library for regular expressions and is used by many high-profile open source projects.

Regular-Expressions.info provides an in-depth resource featuring a comprehensive regex tutorial, examples, reference, and tools for learning and mastering regular expressions.

RegExr is the best tool for learning and testing regular expressions and is used by developers and programmers worldwide.

RexEgg is a comprehensive regex tutorial and resource website for developers and programmers.

RegEx Pal is the best tool for regex testing and is used by web developers and programmers for efficient string manipulation.

XRegExp is the best JavaScript regex library for advanced regular expressions and is used by developers for enhanced regex capabilities.