Working with text, Regular expressions


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This is the ninth set of slightly updated slides from a Perl programming course that I held some years ago.
I want to share it with everyone looking for intransitive Perl-knowledge.
A table of content for all presentations can be found at
The source code for the examples and the presentations in ODP format are on

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Working with text, Regular expressions

  1. 1. Perl Programming Course Working with text Regular expressionsKrassimir
  2. 2. Contents1. Simple word matching2. Character classes3. Matching this or that4. Grouping5. Extracting matches6. Matching repetitions7. Search and replace8. The split operator
  3. 3. Simple word matching• Its all about identifying patterns in text• The simplest regex is simply a word – a string of characters.• A regex consisting of a word matches any string that contains that word• The sense of the match can be reversed by using !~ operator my $string =some probably big string containing just about anything in it; print "found stringn" if $string =~ /string/; print "it is not about dogsn" if $string !~ /dog/;
  4. 4. Simple word matching (2)• The literal string in the regex can be replaced by a variable• If matching against $_ , the $_ =~ part can be omitted: my $string =stringify this world; my $word = string; my $animal = dog; print "found $wordn" if $string =~ /$word/; print "it is not about ${animal}sn" if $string !~ /$animal/; for(dog,string,dog){ print "$wordn" if /$word/ }
  5. 5. Simple word matching (3)• The // default delimiters for a match can be changed to arbitrary delimiters by putting an m in front• Regexes must match a part of the string exactly in order for the statement to be true my $string =Stringify this world!; my $word = string; my $animal = dog; print "found $wordn" if $string =~ m#$word#; print "found $word in any casen" if $string =~ m#$word#i; print "it is not about ${animal}sn" if $string !~ m($animal); for(dog,string,Dog){ local $=$/; print if m|$animal| }
  6. 6. Simple word matching (4)• perl will always match at the earliest possible point in the string my $string =Stringify this stringy world!; my $word = string; print "found $word in any casen" if $string =~ m{$word}i;• Some characters, called metacharacters, are reserved for use in regex notation. The metacharacters are (14): { } [ ] ( ) ^ $ . | * + ? • A metacharacter can be matched by putting a backslash before it print "The string n$stringn contains a DOTn" if $string =~ m|.|;
  7. 7. Simple word matching (5)• Non-printable ASCII characters are represented by escape sequences• Arbitrary bytes are represented by octal escape sequences use utf8; binmode(STDOUT, :utf8) if $ENV{LANG} =~/UTF-8/; $=$/; my $string ="containsrn Then we have sometttabs. б"; print matched б(x{431}) if $string =~ /x{431}/; print matched б if $string =~/б/; print matched rn if $string =~/rn/; print The string was:" . $string.";
  8. 8. Simple word matching (6)• To specify where it should match, use the anchor metacharacters ^ and $ . use strict; use warnings;$=$/; my $string =A probably long chunk of text containing strings; print matched "A" if $string =~ /^A/; print matched "strings" if $string =~ /strings$/; print matched "A", matched "strings" and something in between if $string =~ /^A.*?strings$/;
  9. 9. Character classes• A character class allows a set of possible characters, to match• Character classes are denoted by brackets [ ] with the set of characters to be possibly matched inside• The special characters for a character class are - ] ^ $ and are matched using an escape• The special character - acts as a range operator within character classes so you can write [0-9] and [a-z]
  10. 10. Character classes• Example use strict; use warnings;$=$/; my $string =A probably long chunk of text containing strings; my $thing = ong ung ang enanything; my $every = iiiiii; my $nums = I have 4325 Euro; my $class = dog; print matched any of a, b or c if $string =~ /[abc]/; for($thing, $every, $string){ print ingy brrrings nothing using: .$_ if /[$class]/ } print $nums if $nums =~/[0-9]/;
  11. 11. Character classes• Perl has several abbreviations for common character classes • d is a digit – [0-9] • s is a whitespace character – [ trnf] • w is a word character (alphanumeric or _) – [0-9a-zA-Z_] • D is a negated d – any character but a digit [^0-9] • S is a negated s; it represents any non-whitespace character [^s] • W is a negated w – any non-word character • The period . matches any character but "n" • The dswDSW inside and outside of character classes • The word anchor b matches a boundary between a word character and a non-word character wW or Ww
  12. 12. Character classes• Example my $digits ="here are some digits3434 and then "; print found digit if $digits =~/d/; print found alphanumeric if $digits =~/w/; print found space if $digits =~/s/; print digit followed by space, followed by letter if $digits =~/ds[A-z]/;
  13. 13. Matching this or that• We can match different character strings with the alternation metacharacter |• perl will try to match the regex at the earliest possible point in the string my $digits ="here are some digits3434 and then "; print found "are" or "and" if $digits =~/are|and/;
  14. 14. Extracting matches• The grouping metacharacters () also allow the extraction of the parts of a string that matched• For each grouping, the part that matched inside goes into the special variables $1 , $2 , etc.• They can be used just as ordinary variables
  15. 15. Extracting matches• The grouping metacharacters () allow a part of a regex to be treated as a single unit• If the groupings in a regex are nested, $1 gets the group with the leftmost opening parenthesis, $2 the next opening parenthesis, etc. my $digits ="here are some digits3434 and then678 "; print found a letter followed by leters or digits":.$1 if $digits =~/[a-z]([a-z]|d+)/; print found a letter followed by digits":.$1 if $digits =~/([a-z](d+))/; # $1 $2 print found letters followed by digits":.$1 if $digits =~/([a-z]+)(d+)/; # $1 $2
  16. 16. Matching repetitions• The quantifier metacharacters ?, * , + , and {} allow us to determine the number of repeats of a portion of a regex• Quantifiers are put immediately after the character, character class, or grouping • a? = match a 1 or 0 times • a* = match a 0 or more times, i.e., any number of times • a+ = match a 1 or more times, i.e., at least once • a{n,m} = match at least n times, but not more than m times • a{n,} = match at least n or more times • a{n} = match exactly n times
  17. 17. Matching repetitionsuse strict; use warnings;$=$/;my $digits ="here are some digits3434 and then678 ";print found some letters followed by leters ordigits":.$1 .$2if $digits =~/([a-z]{2,})(w+)/;print found three letter followed by digits":.$1 .$2if $digits =~/([a-z]{3}(d+))/;print found up to four letters followed by digits":.$1 .$2if $digits =~/([a-z]{1,4})(d+)/;
  18. 18. Matching repetitions• Greeeedy use strict; use warnings;$=$/; my $digits ="here are some digits3434 and then678 "; print found as much as possible letters followed by digits":.$1 .$2 if $digits =~/([a-z]*)(d+)/;
  19. 19. Search and replace• Search and replace is performed using s/regex/replacement/modifiers.• The replacement is a Perl double quoted string that replaces in the string whatever is matched with the regex .• The operator =~ is used to associate a string with s///.• If matching against $_ , the $_ =~ can be dropped.• If there is a match, s/// returns the number of substitutions made, otherwise it returns false
  20. 20. Search and replace• The matched variables $1 , $2 , etc. are immediately available for use in the replacement expression.• With the global modifier, s///g will search and replace all occurrences of the regex in the string• The evaluation modifier s///e wraps an eval{...} around the replacement string and the evaluated result is substituted for the matched substring.• s/// can use other delimiters, such as s!!! and s{}{}, and even s{}//• If single quotes are used s, then the regex and replacement are treated as single quoted strings
  21. 21. Search and replace• Example #TODO....
  22. 22. The split operator• split /regex/, string splits string into a list of substrings and returns that list• The regex determines the character sequence that string is split with respect to #TODO....
  23. 23. Regular expressions• Resources • perlrequick - Perl regular expressions quick start • perlre - Perl regular expressions • perlreref - Perl Regular Expressions Reference • Beginning Perl (Chapter 5 – Regular Expressions)
  24. 24. Regular expressionsQuestions?