1. Pattern Matching: Simple Patterns

2. Introduction Programmers often need to scan a file, directory, etc. for a specific substring. –Find all files that begin with “A”. –Find all files that end in “txt” This capability is provided by a variety of tools. –e.g. egrep, grep, awk, Useful to include this functionality in a programming language.

3. Perl’s Pattern Matcher Perl has a built in pattern matcher. –Motivation: system administrators frequently use regular expressions. They also use Perl. Syntax is borrowed from the grep utility in Unix. Based on regular expressions from computer science.

4. Perl’s Pattern Matcher (cont.) Operates over a single string. Contexts: –Scalar: Returns true or false. –List: Matching substrings returned in a list. The syntax is: m dl pattern dl [modifiers] (/) is the most common delimiter. –m operator is unnecessary. Other delimiters can be used: m~pattern~

5. Simple Patterns Simple patterns – match individual characters or character classes. An abstract representation of a set of strings. A pattern “matches” when the string it’s compared with is in the set. Matching is done from left to right.

6. Three Categories of Characters Normal characters: –Match themselves. –Includes escape characters – e.g. \t, \cC Metacharacters: –Have special meanings in patterns –\ | ( ) [ ] { } ^ $ * + Period: –Matches any character except newline.

7. An Example $_ = “It’s snowing today.”; if (/snow/) { print “There was snow somewhere in $_”; } else { print “$_ was snowless \n”; }

8. Character Classes Character classes specify collections of characters in patterns. Defined by placing the set in [ ] –e.g. /[<>=] Dashes are used specify ranges of characters: –/[A-Za-z]/ –/[0-7]/ –/[0-3-]/

9. Exclusion From a Class Characters can be excluded from a class with (^) Matches anything except the specified characters. For example: –/[^A-Za-z]/ –/[^01]/

10. Useful Abbreviations AbbreviationPatternMatches \d[0-9]A digit \D[^0-9]A nondigit \w[A-Za-z_]A word char \W[^A-Za-z_]A nonword char \s[ \r\t\n\f]A white-space char \S[^ \r\t\n\f]A non-white-space char

11. Some Examples /[A-Z]”\s/ /[\dA-Fa-f]/ /\w\w:\d\d/ /0x\d/

12. Variables in Patterns A variable in a pattern is interpolated. For example, $hexpat = “\\s[\dA-Fa-f]\\s”; if (/$hexpat/) { print “$_ has a hex digit.” }

13. Quantifiers Quantifiers can make a pattern more powerful. Allows a pattern to be repeated a specified number of times. Perl has four kinds of quantifier: –*, +, ?, {m, n} Quantifier immediately follows the pattern it quantifies.

14. {m, n} {n} – exactly n repetitions. {m,} – at least m repetitions. {m,n} – at least m, but not more than n repetitions.

15. {m,n} Examples /a{1,3}b/ - ab, aab, aaab /ab{3}c/ - abbbc /ab{2,}c/ - abbc, abbbc, abbbbc, … /c{3} z{5}/ - ccc zzzzz /[abc] {1, 2}/ - a,b,c,ab,ac,ba,bc,ca,cb

16. Asterisk (*) (*) means zero or more repetitions. Equivalent to {0,} For example, –/0\d\d*/ –/\w\w*/ –/bob.*cat/

17. Plus (+) (+) means one or more repetitions. Equivalent to {1,} For example, –/\w+/ –/[A-Za-z][A-Za-z\d_]+/ –/\d+\.\d+/

18. Question Mark (?) (?) means either zero or one. Equivalent to {0,1}. For example, –/\d+\.?/ –/\$?\d+\.\d\d/ –/”?\w+”?/

19. Subpatterns Quantifiers modify only the last character. –e.g. /ball*/ () can be used to group parts of patterns. The quantifier modifies the group. For example, –/(ball)*/ –/(boo! ){3}/

20. Alternation (|) is the logical OR operator in a pattern. /a|e|i|o|u/ is equivalent to /[aeiou]/ For example, –/(Bob|Tom|Pussy|Scaredy)cat/ –/t(oo?|wo)/ Be careful! –/Tom|Tommie/

21. Precedence The precedence of the operators are: –Parenthesis –Quantifiers –Character Sequence –Alternation For example, –/#|-+/ –/(#|-)+/