Exceptions I

re1.rb

Regular Expressions II

# Get a chomped string, or nil at EOF.
def getstr
    print "Please enter a test string: "
    str = gets
    return str unless str
    return str.chomp
end

# Test strings
while str = getstr
    print "You entered: ", str, "\n"

    # Run some random tests and print a descriptive message for ones which
    # match.
    num = 0
    if str =~ /^\s*$/ then
        print "  > Your string is all blanks.\n"
        next
    end
    if str =~ /Mommy/ then
        print "  > Contains Mommy\n"
        num += 1 
    end
    if str =~ /Mommy.*Daddy/ then
        print "  > Contains Mommy, then Daddy\n"
        num += 1 
    end
    if str !~ /CAT/ then
        print "  > Does not contain CAT.\n"
        num += 1 
    end
    if str !~ /[Cc][Aa][Tt]/ then
        print "  > Does not contain cat (any capitalization).\n"
        num += 1 
    end
    if str =~ /^AA/ then
        print "  > Starts with AA\n"
        num += 1 
    end
    if str =~ /(ing|ed)$/ then
        print "  > Ends in ing or ed\n"
        num += 1 
    end
    if str =~ /^\d+$/ then
        print "  > Is an unsigned integer\n"
        num += 1 
    end
    if str =~ /^(\+|\-)\d+$/ then
        print "  > Is a signed integer\n"
        num += 1 
    end
    if str !~ /[AEIOUaeiou]/ then
        print "  > Contains no vowels.\n"
        num += 1
    end
    if str =~ /@[^A-Z]*$/ then
        print "  > Has an at sign with no upper case letters following it.\n"
        num += 1
    end
    if str =~ /^[^%]*%[^%]*%[^%]*%[^%]*$/ then
        print "  > Contains exactly 3 percent signs.\n" 
        num += 1
    end
    if str =~ %r=^(http|ftp)://([a-zA-Z-]+(\.[a-zA-Z-]+)+)(/|$)= then
        proto = $1
        host = $2
        print "  > Looks like a lot of common URLs with protocol #{proto} ",
                "and host #{host}.\n"
        num += 1
    end

    # What happened?
    if num == 0 then
        print "=== That string is remarkably boring. ===\n"
    else
        print "=== Found ", num,
            " interesting thing" + (if num > 1 then "s" else "" end),
            " about that string. ===\n"
    end
end

---

Regular expressions are patterns which can be matched against strings. The usual syntax is to surround them with slashes, which create a Regexp object, much as putting quoted characters create a String object. As the %Q for strings, the %r notation allows an alternate delimiter on regular expressions.

In this program, use the =~ operator to ask if a string matches the pattern, and !~ to ask if it doesn't.

Parentheses used in regular expressions group operations just as the do in ordinary expressions. In addition, the characters matched by each parenthetical group are assigned to a special variable, $n, where the left paren was the nth from the left.

Regular expressions are a large topic, and we'll have some class discussion about them.

Exceptions I

Regular Expressions II