source: downloads/boost_1_34_1/libs/xpressive/doc/preface.qbk @ 29

[section Preface]

[:['Wife:]      It's a floor wax!\n
  ['Husband:]   No, it's a dessert topping!\n
  ['Announcer:] Stop! You're both right. It's a floor wax ['and] a dessert topping!]
[:[*['-- Saturday Night Live]]]

[h2 Description]

xpressive is an advanced, object-oriented regular expression template library for C++.
Regular expressions can be written as strings that are parsed at run-time, or as expression
templates that are parsed at compile-time. Regular expressions can refer to each other and
to themselves recursively, allowing you to build arbitrarily complicated grammars out of
them.

[h2 Motivation]

If you need to manipulate text in C++, you have typically had two disjoint options: a regular
expression engine or a parser generator. Regular expression engines (like _regexpp_) are powerful
and flexible; patterns are represented as strings which can be specified at runtime. However,
that means that syntax errors are likewise not detected until runtime. Also, regular expressions
are ill-suited to advanced text processing tasks such as matching balanced, nested tags. Those
tasks have traditionally been handled by parser generators (like the _spirit_fx_). These
beasts are more powerful but less flexible. They generally don't allow you to arbitrarily modify
your grammar rules on the fly. In addition, they don't have the exhaustive backtracking semantics
of regular expressions, which can make it more challenging to author some types of patterns.

xpressive brings these two approaches seamlessly together and occupies a unique niche in the
world of C++ text processing. With xpressive, you can choose to use it much as you would use
_regexpp_, representing regular expressions as strings. Or you can use it as you would use _spirit_,
writing your regexes as C++ expressions, enjoying all the benefits of an embedded language
dedicated to text manipulation. What's more, you can mix the two to get the benefits of
both, writing regular expression ['grammars] in which some of the regular expressions are 
statically bound -- hard-coded and syntax\-checked by the compiler \-- and others are dynamically
bound and specified at runtime. These regular expressions can refer to each other recursively,
matching patterns in strings that ordinary regular expressions cannot.

[h2 Influences and Related Work]

The design of xpressive's interface has been strongly influenced by John Maddock's
_regexpp_ library and his _proposal_
to add regular expressions to the Standard Library. I also drew a great deal of
inspiration from Joel de Guzman's _spirit_fx_, which served as the model
for static xpressive. Other sources of inspiration are the _perl6_ redesign and _greta_.
(You can read a summary of the changes Perl 6 will bring to regex culture
[@http://dev.perl.org/perl6/doc/design/syn/S05.html here].)

[endsect]