CS 230 Programming Languages

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Archibald James
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1 CS 230 Programming Languages 09 / 20 / 2013 Instructor: Michael Eckmann

2 Today s Topics Questions/comments? Continue Regular expressions

3 Matching string basics =~ (matches) m/ / (this is the format of match regular expression) Note: you put the regular expression in between the / / The format of a complete expression which will evaluate to true or false depending on whether the string matched or not (anywhere within the string) is as follows: ($some_string =~ m/regexprtomatch/) Examples: ( CS230 Programming Languages =~ m/prog/) ( CS230 Programming Languages =~ m/cs106/) # true # false

4 Matching string basics!~ (doesn't match) Examples: ( CS230 Programming Languages!~ m/prog/) # false ( CS230 Programming Languages!~ m/cs106/) # true You may use any other character as a delimiter other than the /'s if you wish. e.g. m#prog# could be used instead of m/prog/ The m for the m/ / matching operator is optional if we use the /'s as the delimiters. To match a / you have two choices, either use a different delimiter, then just put the slash in there, or backslash the slash to make it an escape sequence.

5 Variables can be put inside the / / search pattern and they are interpolated. =~ can be omitted if matching the $_ special default variable Metacharacters {}[]()^$. *+?\ have special meaning inside the m/ /. If you wish to match those characters you need to backslash them. Also, regardless of the delimiter, if you wish to match what you used as the delimiter, guess what you need to do? ^ forces the match to be required to be at the very beginning of the string $ forces the match to be required to be at the very end of the string Examples: ( CS230 Programming Languages =~ m/^prog/) # false, not at beginning ( CS230 Programming Languages =~ m/ges$/) # true, ges is at the end ( CS230 Programming Languages =~ m/^ges$/) # false, ges not the whole string ( CS230 =~ m/^cs230$/) # true, CS230 is the complete string

6 Character classes are put between [ ] square brackets within the pattern --- this means any one single character in the [ ] is able to match. May use ^ (not) to not match a char inside the character class May use (hyphen) to specify a range of characters for the class Hay is for horses =~ m/[a-z]ay/; # matches if the string contains any capital letter followed by ay The boardgame Payday is lame =~ m/[a-z]ay/; # this would be true too, it matches Pay The boardgame Payday is lame =~ m/[^a-z]ay/; # this would be true too, why?

7 Character classes are put between [ ] square brackets within the pattern --- this means any one single character in the [ ] is able to match. May use ^ (not) to not match a char inside the character class May use (hyphen) to specify a range of characters for the class Hay is for horses =~ m/[a-z]ay/; # matches if the string contains any capital letter followed by ay The boardgame Payday is lame =~ m/[a-z]ay/; # this would be true too, it matches Pay The boardgame Payday is lame =~ m/[^a-z]ay/; # this would be true too, why? These are special variables that are set for a match: $` $& and $' (left, matched, right) Let's put this code in a program and print out the variable values to see what matches.

8 There are ways to specify common character classes \d (any digit) \s (any whitespace \ \t\r\n\f ) \w (any word character (a digit, letter or underscore)) \D (any non-digit) \S (any non-whitespace) \W (any non-word character). (any character other than newline \n) These can be used within the square brackets or outside of them.

9 Modifiers are characters that go after the second forward slash i is a modifier for ignore case. The behavior for no modifier (the default) is that. Matches any non-newline character ^ matches at beginning of string $ matches at end of string (or before a newline at end) s modifier: treats the string as a single long line, so. matches any character including newline m modifier: treats string as multiple lines so, ^ and $ match the beginning or end of any line But now, \A matches the beginning of the whole string, \Z matches the end of the whole string. Let s look at this webpage s examples under Using Character Classes for some more examples:

10 alternation character (acts sort of like a logical or) Grouping characters using the parentheses Getting the submatches by using the $1, $2, $3, etc. variables which are set via the parentheses. Repetitions? - 0 or 1 time * - 0 or more times or more times { } min and max, at least or exactly Let s continue looking at this site for examples:

11 In terms of regular expression repetition quantifiers, what does greedy mean? A quantifier is greedy if it matches as much of the string as possible while still allowing the whole regular expression to match. We'll see that greediness in action now. Let s continue looking at this site for examples of matching repetitions and the 4 principles that are followed:

12 Recap on the special variables we learned $_ $` $& and $' (left, match, right) $0 (program name) $1, $2, $3,... (the submatches)

13 Let's write a few regular expressions. match any signed or unsigned integers of arbitrary length. e.g. it should match but not things like: abcd etc.

14 Let's try these: 1) ignore beginning whitespace if there is any, and match the word program and store the rest of the string (after the word program) into some variable. 2) Now what if there were \n's in the string? What might we change? 3) cs230 or cs106 or CS106 or CS230 but not Cs230, or cs106 etc.

15 1) ignore beginning whitespace if there is any, and match the word program and store the rest of the string (after the word program) into some variable. m/\s*program(.*)/ 2) Now what if there were \n's in the string? What might we change? m/\s*program(.*)/s 3) cs230 or cs106 or CS106 or CS230 but not Cs230, or cs106 etc. m/cs230 cs106 CS230 CS106/ OR m/(cs CS)( )/

16 Let s look at a larger parsing example using many of the features we just learned: The doing string selections section of: The following page is a good page for reference. It is a nice summary of the different characters and their meanings with succinct examples:

Perl Regular Expressions. Perl Patterns. Character Class Shortcuts. Examples of Perl Patterns

Perl Regular Expressions. Perl Patterns. Character Class Shortcuts. Examples of Perl Patterns Perl Regular Expressions Unlike most programming languages, Perl has builtin support for matching strings using regular expressions called patterns, which are similar to the regular expressions used in