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Regular expressions are very powerful feature, however they are often difficultto interpret. Rex allows you to build complex regular expressions from humanreadable expressions. So instead of writing (and later trying to decipher)

r<-"^(?:(((?:[^:])+)://))?((?:(?:(?!:/).)*)+)(?:(:([[:digit:]]+)))?(?:(/.*))?$"

You can write

r<- rex(start,## match the protocol -- may exist or may not  maybe(capture(      capture(except_some_of(":")),"://"      )),## match the path  capture(one_or_more(not(":/"))),## get the port  maybe(capture(":", capture(numbers))),## and the rest  maybe(capture("/",anything)),end)

While these expressions are a bit longer than their corresponding regularexpression, they are much more readable and maintainable.

install.packages("rex")

The vignettes have longer form usage examples.

• URL Validation
• Webserver Log Parsing

Eachrex() function call can include a number of functions and shortcuts.For a full list of the functions available please see?rex and?shortcuts.

Rex functions are not exported because they are only useful withinrex()calls, but they can be temporarily attached usingrex_mode() which allowsthem to be auto-completed.

Usingrex in other packages will generate spurious NOTEs fromR CMD checkunless you include a call torex::register_shortcuts() with your package namesomewhere in your package source. This function registers all of the rexshortcuts as valid variables fixing the NOTEs.

• Regularity - Ruby library thatpartially inspiredrex.
• PCRE - Perl Compatible Regular Expressions, theengine thatrex regular expressions use.
• Perl 5 Regular Expressions - Perlregular expression documentation, which are nearly 100% compatible with PCRE.