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 Ruby user's guide Regular expressions  

Let's put together a more interesting program.  This time we test whether a string fits a description, which we will call a pattern.

There are some characters and character combinations that have special meaning in these patterns, including:

[]range specificication (e.g., [a-z] means a letter in the range a to z)
\wletter or digit; same as [0-9A-Za-z]
\Wneither letter or digit
\sspace character; same as [ \t\n\r\f]
\Snon-space character
\ddigit character; same as [0-9]
\Dnon-digit character
\bbackspace (0x08) (only if in a range specification)
\bword boundary (if not in a range specification)
\Bnon-word boundary
*zero or more repetitions of the preceding
+one or more repetitions of the preceding
{m,n}at least m and at most n repetitions of the preceding
?at most one repetition of the preceding; same as {0,1}
|either preceding or next expression may match
()grouping

The common term for patterns that use this strange vocabulary is regular expressions.  In ruby (as in Perl) they are general surrounded by forward slashes rather than double quotes.  If you have never worked with regular expressions before, they probably look anything but regular, but you would be wise to spend some time getting familiar with them.  They have a concise expressive power that will save you headaches (and many lines of code) whenever you need to do pattern matching, searching, or other manipulations on text strings.

For example, suppose we want to test whether a string fits this description: "Starts with lower case f, which is immediately followed by exactly one upper case letter, and optionally more junk after that, as long as there are no more lower case characters."  If you're an experienced C programmer, you've probably already written about a dozen lines of code in your head, right?  Admit it; you can hardly help yourself.  But in ruby you need only request that your string be tested against the regular expression /^f[A-Z](^[a-z])*$/.

How about "Contains a hexadecimal number enclosed in angle brackets"?  No problem.

 ruby> def chab (s)   # "contains hex in angle brackets"
     |    (s =~ /<0(x|X)(\d|[a-f]|[A-F])+>/) != nil
     | end
   nil
 ruby> chab "Not this one."
   false
 ruby> chab "Maybe this? {0x35}"    # wrong kind of brackets
   false
 ruby> chab "Or this? <0x38z7e>"    # bogus hex digit
   false
 ruby> chab "Okay, this: <0xfc0004>."
   true

Though regular expressions can be puzzling at first glance, you will quickly gain satisfaction in being able to express yourself so economically.

Here is a little program to help you experiment with regular expressions.  Store it as regx.rb and run it by typing "ruby regx.rb" at the command line.

 # Requires an ANSI terminal!

 st = "\033[7m"
 en = "\033[m"

 while TRUE
   print "str> "
   STDOUT.flush
   str = gets
   break if not str
   str.chop!
   print "pat> "
   STDOUT.flush
   re = gets
   break if not re
   re.chop!
   str.gsub! re, "#{st}\\&#{en}"
   print str, "\n"
 end
 print "\n"

The program requires input twice, once for a string and once for a regular expression.  The string is tested against the regular expression, then displayed with all the matching parts highlighted in reverse video.  Don't mind details now; an analysis of this code will come soon.

 str> foobar
 pat> ^fo+
 foobar
 ~~~

Note: In the program output, what you see here as red text (foo) appears in reverse video.  the "~~~" is just for text-based browsers.

Let's try several more inputs.

 str> abc012dbcd555
 pat> \d
 abc012dbcd555
    ~~~    ~~~

If that surprised you, refer to the table at the top of this page: \d has no relationship to the character d, but rather matches a single digit. 

What if there is more than one way to correctly match the pattern?

str> foozboozer
pat> f.*z
foozboozer
~~~~~~~~

foozbooz is matched instead of just fooz, since a regular expression maches the longest possible substring.

Here is a pattern to isolate a colon-delimited time field.

 str> Wed Feb  7 08:58:04 JST 1996
 pat> [0-9]+:[0-9]+(:[0-9]+)?
 Wed Feb  7 08:58:04 JST 1996
            ~~~~~~~~

"=~" is a matching operator with respect to regular expressions; it returns the position in a string where a match was found, or nil if the pattern did not match.

 ruby> "abcdef" =~ /d/
    3
 ruby> "aaaaaa" =~ /d/
    nil

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