Building regular expressions in Groovy

Because of compact syntax regular expressions in Groovy are more readable than in Java. Here is how Jeffrey Friedl's example would look like in Groovy:

def subDomain  = '(?i:[a-z0-9]|[a-z0-9][-a-z0-9]*[a-z0-9])' // simple regex in single quotes
def topDomains = """
       (?x-i : com        \\b     # you can put whitespaces and comments
             | edu        \\b     # inside regex in eXtended mode
             | biz        \\b
             | in(?:t|fo) \\b     # but you have to escape
             | mil        \\b     # backslashes in multiline strings
             | net        \\b
             | org        \\b
             | [a-z][a-z] \\b
       )"""

def hostname = /(?:${subDomain}\.)${topDomains}/  // variable substitution in slashy strings

def NOT_IN   = /;\"'<>()\[\]{}\s\x7F-\xFF/  // backslash is not escaped in slashy strings
def NOT_END  = /!.,?/
def ANYWHERE = /[^${NOT_IN}${NOT_END}]/
def EMBEDDED = /[$NOT_END]/                 // you can ommit {} around var name

def urlPath  = "/$ANYWHERE*($EMBEDDED+$ANYWHERE+)*"

def url =
    """(?x:
             \\b

             # match the hostname part
             (
               (?: ftp | http s? ): // [-\\w]+(\\.\\w[-\\w]*)+
             |
               $hostname
             )

             # allow optional port
             (?: :\\d+ )?

             # rest of url is optional, and begins with /
             (?: $urlPath )?
       )"""

assert 'http://www.google.com/search?rls=en&q=regex&ie=UTF-8&oe=UTF-8' ==~ url

As you can see, there are several options, and for every subexpression you can choose the one that's more expressive.

Resources

• Martin Fowler on composed regexes
• Pragmatic Dave on regexes in Ruby
• Feature request to make regexes even groovier
• Mastering Regular Expressions — best regex book
• Groovy Pattern and Matcher classes