Regular Expressions in Perl – Part 2Grouping Things & Hierarchical MatchingContinuedSlide 4Extracting MatchesSlide 6Slide 7Slide 8Matching RepetitionsSlide 10Slide 11ContinuedSlide 13Slide 14Slide 15Slide 16Slide 17Slide 18Building a RegexpSlide 20Slide 21Slide 22Slide 23By: Andrew CoryGrouping Things &Hierarchical MatchingGrouping characters – ( and )Allows parts of a regular expression to be treated as a single unitUseful for the creation of multiple words and/or phrases with similar base characters and/or wordsEx. /house(cat|keeper)/ =~ /housecat|housekeeper/Ex. /(a|[bc])d/ =~ ‘ad’, ‘bd’, or ‘cd’Ex. /(19|20|)\d\d/ =~ matches 19xx, 20xx, or xxContinuedBacktracking: step-by-step process of trying alternatives and seeing if they match, and moving on to the next alternative if it doesn’tAny given regular expression has several paths that result in a different stringBacktracking is a trial-and-error method that goes through one character at a time.ContinuedBacktracking Example – “abcd” =~ /(af|ab)(ce|c|cd)/;1 – start with letter “a”2 – try 1st alternative3 – ‘a’ matches, but ‘f’ doesn’t match ‘b’, backtrack to ‘a’ and try 2nd alternative4 – ‘a’ and ‘b’ matches the first 2 characters, first group satisfied, next group.5 – ‘c’ matches, but ‘e’ doesn’t, backtrack to ‘c’, try 2nd alt.6 – ‘c’ matches, second group is satisfied, therefore whole expression is satisfied by “abcd”Note – 3rd alt. in the 2nd group matches too, but is irrelevant: the string already satisfied the regular expression.Extracting MatchesParentheses not only group, they also extract and separate parts of strings that match the given conditionI.e. if ($time =~ /(\d\d):(\d\d):(\d\d)/) {$hours = $1;$minutes = $2;$seconds = $3; }($hours, $minutes, $second) =($time =~ /(\d\d):(\d\d):(\d\d)/);ContinuedNested grouping in a regular expression results in more separationEx. /(ab(cd|ef)((gi)|j))/;$1 = ab $2 = cd|ef $3 = gi|j $4 = giBackreferences – related to matching variables $1, $2, etc., but can only be used inside the regular expressionUseful for repeating phrasesEx. /(\w\w\w)\1/ =~ ‘booboo’, or ‘murmur’ContinuedPositions of string portions that match the conditions are also stored in the @- and @+ arraysEx. $x = “Mmm…donut”;$x =~ /^(Mmm)\.\.\.(donut)/;Foreach $expr (1..$#-) {print “$expr: ‘${$expr}’ at ($-[$expr],$+[$expr])\n”Output:1: ‘Mmm’ at (0,3)2: ‘donut’ at (6,11)ContinuedStrings that have no groupings but are still searched for are still stored in separate variables$` is the string before the match$& is the string that matched$’ is the string after the matchEx. $x = “I like chips”;$x =~ /like/;$` = “I “ $& = “like” $’ = “ chips”Matching RepetitionsQuantifier characters ?, *, +, and {} are used to match words or syllables of any length without massive amounts of repetitionDefinitionsa? = matches ‘a’ one or zero timesa* = matches ‘a’ any number of timesa+ = matches ‘a’ one or more times (at least once)a{n,m} = matches at least n times, not more than m timesa{n, } = matches at least n or more timesa{n} = matches exactly n timesContinuedExamples/[a-z]+\s+\d*/ = a lowercase word, some space, and any number of digits (ajc 93, jgro 843986)/(\w+)\s\1/ = a doubled word of any length with a space inbetween (jon jon, hidalgo hidalgo)/y(es)?/i = ‘y’, ‘Y’, or ‘yes’ContinuedPerl will always try to match as much of a given string as possible to a regular expression so long as the regular expression holds trueI.e. the ‘?’ operator will be matched to the string with whatever precursor present, if not it stops using itEx. $x = “the cat in the hat”;$x =~ /^(.*)(at)(.*)$/;$1 = ‘the cat in the h’$2 = ‘at’$3 = ‘’ContinuedQuantifiers that grab as much of the string as possible are known as ‘maximal match’ or ‘greedy’ quantifiers4 important regular expression principlesPrinciple 1: any regexp will be matched at the earliest possible position in the stringPrinciple 2: The leftmost alternation that matches in a group will be the one used (a|b|c)Principle 3: Matching quantifiers will match as much of the string as possible while holding true to the regexpPrinciple 4: The leftmost greedy quantifier has more priority over other existing greedy quantifiersContinuedExamples$x = “The programming republic of Perl”;$x =~ /^(.+)(e|r)(.*)$/$1 = ‘The programming republic of Pe’$2 = ‘r’$3 = ‘l’$x =~ /.*(m{1,2})(.*)$/$1 = ‘m’$2 = ‘ing republic of Perl’ContinuedSometimes returning the minimal piece of a string is essential, thus, ‘minimal match’ or ‘non-greedy’ quantifiers ??, *?, +?, and {}? were created.Definitionsa?? = match ‘a’ 0 or 1 times, 0 first, then 1a*? = match ‘a’ any number of times, as few as possiblea+? = match ‘a’ 1 or more times, as few as possiblea{m,n}? = match n times, no more than m, as few as pos.a{n, }? = match n times, as few as possiblea{n}? = match n times, same thing as a{n}ContinuedExamples: same as above, different operators!$x = “The programming republic of Perl”;$x =~ /^(.+?)(e|r)(.*)$/$1 = ‘Th’$2 = ‘e’$3 = ‘ programming republic of Perl’$x =~ /.*?(m{1,2})(.*)$/$1 = ‘mm’$2 = ‘ing republic of Perl’ContinuedNote: Principle 3 (matching quantifiers) may be manipulated for non-greedy quantifiers so that the leftmost quantifier matches the least amount of the string as possibleContinuedQuantifiers are susceptible to backtrackingEx. $x = “the cat in the hat”$x =~ /^(.*)(at)(.*)$/;$1 = ‘the cat in the h’ $2 = ‘at’ $3 = ‘’1 Start with the first letter, ‘t’2 The first quantifier starts, matches whole string3 ‘a’ does not match the end of the string, backtrack once4 ‘a’ does not match the last letter ‘t’, backtrack once more5 match ‘a’, then the ‘t’6 move on to the 3rd element. Already at the end of the string, assign it as an empty stringContinuedError alert!Nested indeterminable quantifiers are dangerous thingsEx. /(a|b+)*/;In the above example, the first repetitions searches with b+ of whatever length (up to infinite), and then again searches with the *
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