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Regular Expressions C++0x Sources

C++0xRegular Expressions

Simon Andreas Frimann Lund

Datalogisk InstitutKøbenhavns Universitet

Maj 16, 2008


Regular Expressions

Regular Expression, regex or regexp for short.”A set of characters, metacharacters, and operators that define astring or group of strings in a search pattern.”

� "regex"(simple regex matching the text ”regex”)

� "[-+]?([0-9]*.[0-9]+|[0-9]+)"(simple regular expression matching... what?)

The set of metacharacters, operators and other features are usuallycalled a regex flavor.


Regular Expressions

Regular Expression, regex or regexp for short.”A set of characters, metacharacters, and operators that define astring or group of strings in a search pattern.”

� "regex"(simple regex matching the text ”regex”)

� "[-+]?([0-9]*.[0-9]+|[0-9]+)"(simple regular expression matching... what?)

The set of metacharacters, operators and other features are usuallycalled a regex flavor.


Flavors

There exist 15+ popular regex flavours in various languages andtools of which only two are standardized:

� The POSIX Standard Basic Regex / Extended Regex.

� GNU BRE / ERE, GNU extensions of the standard used inGNU tools such as grep.

� The languages D, Haskell, .NET, Java, ECMA(JavaScript), Python, Ruby all have their own flavors.

� The languages Perl and Tcl has their own flavors as buildin language constructs.

� Libraries such as PCRE (used in PHP), Boost.Regex,Boost.Xpressive, QT/QRegExp each their own flavor.

� And the list goes on...

How are these these tasty flavours implemented?


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Implementations

Basicly all the different flavours are implemented with a NFA(non-deterministic finite automaton) or DFA.Machine size of M character expression, pattern recognitioncomplexity for an N character sequence of S states.

Algo Machine size ComplexityDFA O(2M ) O(N)

bit-par non-backtracking NFA O(M) ∨ (2M ) O(1 + (S/B))N)non-backtracking NFA O(M) ∨ (2M ) O(SN)

backtracking NFA O(M) O(2N )

Currently many different implementations for C++ exist, somebeing procedural others object oriented. Supporting variousdifferent flavours, but most are simply object oriented wrappers forc libraries.


Flavor

The regex support as of TR1 is an extension of std based onBoost.regex, with the following proposed changes/consequences:

� Default ECMAScript syntax.

� Optional support for POSIX BRE/ERE/awk/grep/egrep/sedsyntax.

� Localization features of POSIX is required since ECMA is notcapable of localization.

� Performance is low, due to rich expression features.

� There are given NO performance guarantees.

� Boost has a way to monitor the runtime complexity ofexpressions and stopping them.

� Customizing the expression syntax with trait classes. Nice!


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Implementation

A full implementation in C++, not a wrapper!

Available in the header file <regex>

Representation:

� basic regex, holder of expressions, looks like abasic string.

� match results, iterator of match results

Methods:

� bool regex match(basic string, basic regex)

� bool regex search(basic string, match results,basic regex)

� basic string regex replace(basic string,basic regex, basic string )


Implementation

A full implementation in C++, not a wrapper!

Available in the header file <regex>

Representation:

� basic regex, holder of expressions, looks like abasic string.

� match results, iterator of match results

Methods:

� bool regex match(basic string, basic regex)

� bool regex search(basic string, match results,basic regex)

� basic string regex replace(basic string,basic regex, basic string )


C++0x Example

#inc l u d e <s t d l i b . h>#inc l u d e <regex>#inc l u d e <s t r i n g >#inc l u d e <i o s t r e a m>

us ing namespace s t d ;

r e g e x e x p r e s s i o n ( ”([0−9]+)(\\−| |$ ) ( . * ) ” ) ;

// p r o c e s s f t p : on s u c c e s s r e t u r n s the f t p r e s pon s e code , and f i l l s// msg wi th the f t p r e s pon s e message .i n t p r o c e s s f t p ( const char* r e s p o n s e , s t d : : s t r i n g * msg ){

cmatch what ;i f ( r e g e x m a t c h ( r e s p o n s e , what , e x p r e s s i o n ) ){

// what [ 0 ] c o n t a i n s the whole s t r i n g// what [ 1 ] c o n t a i n s the r e s pon s e code// what [ 2 ] c o n t a i n s the s e p a r a t o r c h a r a c t e r// what [ 3 ] c o n t a i n s the t e x t message .i f ( msg )

msg−>a s s i g n ( what [ 3 ] . f i r s t , what [ 3 ] . second ) ;r e t u r n s t d : : a t o i ( what [ 1 ] . f i r s t ) ;

}// f a i l u r e d i d not matchi f ( msg )

msg−>e r a s e ( ) ;r e t u r n −1;

}

How is C++0x different from C++?


C++ Example

#inc l u d e <s t d l i b . h>#inc l u d e <b o o s t / r e g e x . hpp>#inc l u d e <s t r i n g >#inc l u d e <i o s t r e a m>

us ing namespace b o o s t ;

r e g e x e x p r e s s i o n ( ”([0−9]+)(\\−| |$ ) ( . * ) ” ) ;

// p r o c e s s f t p : on s u c c e s s r e t u r n s the f t p r e s pon s e code , and f i l l s// msg wi th the f t p r e s pon s e message .i n t p r o c e s s f t p ( const char* r e s p o n s e , s t d : : s t r i n g * msg ){

cmatch what ;i f ( r e g e x m a t c h ( r e s p o n s e , what , e x p r e s s i o n ) ){

// what [ 0 ] c o n t a i n s the whole s t r i n g// what [ 1 ] c o n t a i n s the r e s pon s e code// what [ 2 ] c o n t a i n s the s e p a r a t o r c h a r a c t e r// what [ 3 ] c o n t a i n s the t e x t message .i f ( msg )

msg−>a s s i g n ( what [ 3 ] . f i r s t , what [ 3 ] . second ) ;r e t u r n s t d : : a t o i ( what [ 1 ] . f i r s t ) ;

}// f a i l u r e d i d not matchi f ( msg )

msg−>e r a s e ( ) ;r e t u r n −1;

}

It’s regex replace(”std”, sourceCode, ”boost”) different..


Sources

The C++ Standards Committee, n1429

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n1429.pdf

Wikipedia, C++0x

http://en.wikipedia.org/wiki/C++0x

Regular Expressions

http://www.regular-expressions.info

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n1429.pdf

http://en.wikipedia.org/wiki/C++0x

http://www.regular-expressions.info