Perl

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Perl

• Regular expression string manipulation

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substr function

• string substr(string2,start pos (starts with
  0), offset)
• returns a substring after the start point to
  offset
• string2 is not changed
• str2 "Hi There"
• str substr(str2, 3, 2)
• str "Th" from 4 position to 5 position
• substr(string,start pos, offset) string2
• puts string2 after the start pos and removing old
  string characters to offset.
• str2 "Hi There" str "hi"
• substr(str2, 3,3) str insert and replace
• str2 "Hi hire"
• substr(str2, 3,0) str insert only.
• str2 "Hi hihire"

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index and rindex

• index string, substring , offset
• returns the position before the substring in
  string, else -1
• with offset, position after the offset, else -1
• rindex string, substring , offset
• return the last occurrence of the substring, else
  -1
• with offset, the right most position that may be
  returned.
• pos index str, str2
• returns the position where str2 is found in str

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example of substr and index

• str "There there Jim"
• sstr "Jim"
• replace "Fred"
• substr(str,(index str,sstr),3) replace
• replace Jim with Fred in str
• str "There there Fred"
• The substitution operator is an easier way to do
  this.

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grep

• LIST grep EXPR, LIST
• LIST grep BLOCK LIST
• like map, each element is assigned to the _,
  then processed by BLOCK or EXPR, results are put
  into the list.
• _at_new grep /a-zA-Z/, _at_lines
• NOTE altering _ will alter the original list
• _at_list qw(barney fred dino wilma)
• _at_greplist grep s/bfd// _at_list
• _at_greplist "arney", "red", "ino"
• _at_list "arney", "red", "ino", "wilma"

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s/// Operator (Substitution)

• str s/pattern to match/replacement/
• find the first match and replace it
• str s/pattern to match/replacement/g
• Find all matches and replace each of them.
• Simple substitution
• str "3 dogs bit 1 dog"
• str s/dog/cat/
• str "3 cats bit 1 dog"
• str s/dog/cat/g
• str "3 cats bit 1 cat"

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s/// Operator (Substitution) (2)

• s/pattern//
• remove the pattern found
• str "abad"
• s/a//g
• str "bd"
• From substr and index slide
• str s/sstr/replace/
• OR
• str s/Jim/Fred/

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case insensitive substitution

• /i ignore case
• str "Dog, dog, dOg"
• s/DOG/cat/ig
• str "cat, cat, cat"
• str "Dog, dog, dOg"
• s/DOG/cAt/ig
• str "cAt, cAt, cAt"
• The replacement string is replaced as written.

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examples

• str "fred xxx barney"
• str s/x/boom/
• str "fred boomxx barney"
• str s/x/boom/g
• str "fred boomboomboom barney"
• str s/x/boom/
• str "fred boom barney"

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alternation and group matching

• allows an or'd matching
• str "Wilma Flintstone"
• str s/FredWilmaPebbles/Dino/g
• str "Dino Flintstone"
• Replace all instances of Fred or Wilma or Pebbles
  with Dino.
• str "1st time winner"
• str s/(1st2nd3rd) time/Last place/
• 1 is the match, "1st time"
• str "Last place winner"

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single character substitution

• Using
• str s/abc/d/ sub a, b, or c with d
• str s/Fred/x/g
• If str was "Fred", after it would be "xxxx"
• str s/aeiouAeiou/_/g
• replace any non-vowel with an _
• Common mistake
• str s/a-z/A-Z/g
• Should replaces any lower case letter with upper
  case letters but replace side is literal (not a
  pattern)
• if str "hi", then it would be "A-ZA-Z"
• NOTE str uc str upper cases a string.

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matching quantifiers

• str s/a3/b/
• first instance of aaa is replace with b
• str "aaaaa" use this for the rest of the
  slide
• str s/a3,/b/ max matching
• str "b"
• str s/a3,?/b/ min matching
• str "baa" only sub 3 to make a min match
• str s/(a3,?)(a)/b/
• str "b" 1 "aaa" 2 "aa"
• str s/(a3,)(a)/b/
• str "b" 1 "aaaaa" 2 ""
• str s/(a3,?)(a?)/b/ min match on both
• str "baa" 1 "aaa" 2 ""

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matching quantifiers (2)

• str "aaaaab" use this for the rest of the
  slide
• str s/a3,?b/c/
• str "c", why? in order to make the match, it
  used all the a's to include the b.
• 1 or more and ? 0 or 1 time (max match)
• str s/(a)(b?)/c/
• str "c", 1 "aaaaa" and 2 "b"
• str s/(a?)(b??)/c/ min match
• str "caaaab" 1 "a" 2 ""

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matching quantifiers (3)

• Example and perl doesnt always do what you
  think.
• str "ddogg"
• str s/d.g/cat/
• str "cat" max match, makes sense
• str "ddogg"
• str s/d.?g/cat/
• str "catg" min match, but not the best min
  match it can make.

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matching quantifiers (4)

• More Examples (with _ variable)
• _ "a xxx c xxxxx c xxx d"
• s/x1,/d/g produces "a d c d c d d"
• s/x1,?/d/g produces "a ddd c ddddd c ddd d"
• s/x1,2/d/g prodcues "a dd c ddd c dd d"
• s/x1,3/d/g produces "a d c dd c d d"
• s/x2,2/d/g produces "a dx c ddx c dx d"
• or s/x2/d/g

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Anchoring

• str "Fred Flintstone Fred"
• str s/Fred/Wilma/g
• Replaces all instances of Fred with Wilma
• str s/Fred/Wilma/g
• Only the last instance, "Fred Flintstone Wilma",
  even with /g flag
• str s/Fred/Wilma/g
• only the first instance, "Wilma Flintstone Fred",
  even with the /g flag
• str "abcd"
• str s/abc/d/
• str "dd"

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Parentheses as memory

• s/a(.)b(.)c\2d\1/a mess/
• "adbecedd" is converted to "a mess"
• "adbecdde" is not converted.
• s/a(.)b\1c/a mess/
• "addbddc" changes to "a mess"
• "adddbddc" is not changed
• To kept the pattern found use \1 ..\9 in
  replacement
• s/a(.)b\1c/What is this \1/
• "addbddc" converted to "What is this dd"
• again 1 "dd"

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metasymbols

• a very common substitution
• s/\s/ /g replace all whitespace with single
  space.
• " a b\t c" changes to " a b c"
• remove word character duplicates
• str "11aabbdccaa"
• str s/(\w)\1/\1/g
• str "1abcda"
• Remove any duplicates
• str "11 ,,aa"
• str s/(.)\1/\1/g
• str "1 ,a"

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Exercise 10

• What is the outcome of the following
  substitutions? Use _ "ad dog cd"
• s/dog//
• while (/ /) s/ / /g
• s/(\w)\s(\w)/2 1/g
• s/(.)d/Dd/g
• s/(.?)d/Dd/g
• s/(\S)/\1/g
• Write a substitution to change each vowel to an X.

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s/// flags

• like the match operator
• /m let and match next to embedded \n
• /s let . match newline
• /x ignore whitespace and permit comments
• s/// flags only
• /g replace globally, ie all occurrences
• /e evaluate the right side as an expression
• in other words, perl interrupts the right side as
  perl code, where you have return value

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/e flag

• s/(\d)/sprintf("x",1)/ge
• covert all numbers to hex
• "2581" would converted to "0xb23"
• return to the leap year with a trinary operator
• s/(\d)/
• 1 4 ? "1 (not a leap year)"
• 1 100 ? "1 (a leap year)"
• 1 400 ? "1 (not a leap year)"
• "1 (a leap year)"
• /gxe
• "2000" changed to "2000 (a leap year)"

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tr/// Operator (Transliteration)

• same as sed, can as use y/// instead of tr///
• DOES NOT use pattern matching, instead it scans
  character by character and replaces each
  occurrence of a character with a replacement
• tr/SEARCHLIST/REPLACEMENTLIST/cds
• Example
• str "AABBCCDDEE"
• str tr/ABC/XYZ/
• str "XXYYZZDDEE"
• str tr/DE/!/ if the replacement list is to
  short, uses the last one as many times as needed.
• str "XXYYZZ!!!!"

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tr/// Operator (Transliteration) (2)

• Duplicates in the Searchlist are ignored
• str "AABBCCDDEE"
• str tr/AAB/xyz/
• str "xxzzCCDDEE"
• /c means letters not in the Searchlist
• str "AABBCCDDEE"
• str tr/ABC/x/c
• str "AABBCCxxxx"

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tr/// Operator (Transliteration) (3)

• /d delete found, but non-replaced characters
• Changes tr, so if your replacementlist is short,
  those characters are removed
• str "AABBCCDDEE"
• str tr/ABC/xy/d
• str "xxyyDDEE"
• str tr/DE//d
• str "xxyy"

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tr/// Operator (Transliteration) (4)

• /s removes duplicates in replaced characters
• str "AABBCCDDEE"
• str tr/ABC/xyz/s
• str "xyzDDEE"
• tr/// returns the number of characters
  found/replaced.
• count (str tr/ABC/xyz/)
• count 6 str "xxyyzzDDEE"
• count (str tr/ABC//)
• count 6 str "AABBCCDDEE"
• No replacement list, so it just counted them and
  made no replacements. Note s/// would have
  removed them.

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More tr/// Examples

• str "AABBCCDDEE"
• str tr/D//d delete found characters
• str "AABBCCEE"
• str "AABBCCDDEE"
• str tr/ABD/xy/ds delete D, sub A for x and
  B for y and remove duplicates replacements
• str "xyCCEE"
• str tr/a-zA-Z//dc
• remove any non letters from str.
• str tr/A-Za-z/N-ZA-Mn-za-m/
• rotate the characters by 13 letters for simple
  encryption.

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Exercise 11

• What is the outcome of the following
  transliteration? Use _ "fred and barney"
• tr/abcde/ABCDE/
• tr/a-z/ABCDE/d
• count tr/a-z/A-Z/
• tr/a-z/_/c
• tr/a-m/X/s
• tr/aeiou/X/cs
• count tr/aeiou//c
• Change the letters bdr to X and count the number
  of changes.

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Q
A