Andy Little wrote:
Hi Eric,
Boost already has Regex. What is different about xpressive?
regards Andy Little
Although xpressive's primitives are regex-ish in nature, xpressive is far more than just a regular expression engine. (The docs don't make this point strongly enough, my bad.) xpressive is a pattern matching engine with the power of context-free grammars. By allowing you to embed one pattern in another by reference, xpressive lets you find patterns in data that are recursive in nature. Problems that are intractible with "classic" regular expressions, such as matching balanced nested parentheses or matching XML tags, are simple with xpressive. So xpressive is more rightly thought of as a context-free grammar engine with exhaustive backtracking. The goal is to provide one framework and one set of pattern matching primitives that scales from simple strstr-style grepping, through ordinary regexing, all the way to full-blown parsing. That's the primary difference. Other differences flow from that one, like the ability to find patterns in non-char data, and better integration with "real" C++. The latter point is a work in progress, but the ultimate goal is to make it as simple to call C++ code from a regex as it is to call a regex from C++. That includes things like user-defined predicates (extending the set of regex primitives), assignment to variables (string, list, map) from withing the regex, and semantic actions. It's true that there is functional overlap between Regex and xpressive, but xpressive takes a much broader view of pattern matching. And just because one piece of this puzzle has been proposed for standardization doesn't mean that all innovation in this problem space must cease. -- Eric Niebler Boost Consulting www.boost-consulting.com