This document serves as an introductory guide to regular expressions (regex) for beginners, focusing on Perl-compatible regex and offering practical coding examples. It covers various concepts such as matching, substitution, character classes, and quantifiers, as well as offering exercises for application. The guide emphasizes understanding regex through direct practice and application in text manipulation tasks.

1. Regular Expression
Become a Deft
Manipulator of Text Data
Presented by Lambert Lum

2. Beginners Welcome
No previous Perl instruction required.
Practitioners of other languages welcome

3. PCRE
PCRE: Perl compatible Regular Expressions
Available for non-Perl languages.

4. We don’t cover
Infrequently used regex
Optimizations

5. Abbreviations
Regex is a shortened term for regular expression.

6. Perl Refresher
my $var;
$scalar = "jay";
@list = ($scalar, "leno");

7. Jay Leno
Q: Who made the first American flag?
Actual question from
Jay Leno’s Jaywalker segment

8. Jay Leno
Hint:
Last name begins with 'r',
ends with 'ss', and
has an 'o' in the middle
Answer:
Betsy Ross

9. Regex
$last_name =~ m{^r.*o.*ss$};

10. Regex
$last_name =~ m{^r.*o.*ss$};
ross
rouss
rokess
rodiss
ridoss

11. Symbols
^
$
.
*
.*
Begins with
Ends with
Any Character
Zero or more times
Any Character, zero or more times
$last_name =~ m{^r.*o.*ss$};

12. Quantifier Symbols
*
+
?
{2}
{2,}
{2,4}
Zero or more times
one or more times
Zero or one time
2 times
2 or more times
2 to 4 times

13. Practice #1
Q: which one of the following matches m{^r.*o.*s{2,4}$}
a) ros
b) ross
c) rosss
d) rossss
e) rossssss
f ) answers b,c,d
g) answers b,c,d,e

14. Practice #1
Answer: g) answers b,c,d,e

15. Match, substitute, split
# match
my $last_name = 'ross';
if ($last_name =~ m{^r.*o.*ss$}) {
print "match foundn";
}
# substitution
my $full_name = "betsy ross";
$full_name =~ s{s+}{-};
print "substitute space with dash: $full_namen";
# split
$full_name = "ross, betsy";
my @nomens = split m{,s*}, $full_name;
print join (':', @nomens) . "n";

16. Match, substitute, split
# match
my $last_name = 'ross';
if ($last_name =~ /^r.*o.*ss$/) {
print "match foundn";
}
# substitution
my $full_name = "betsy ross";
$full_name =~ s/s+/-/;
print "substitute space with dash: $full_namen";
# split
$full_name = "ross, betsy";
my @nomens = split /,s*/, $full_name;
print join (':', @nomens) . "n";

17. // vs {}
Traditional: //
Most common. Not so good if matching for '/'
e.g. $str =~ ///;
Better: {}
Nesting brace characters in regex do not need escaping

18. Practice #2
Using regex, remove all spaces from the beginning and end of a
string. Hint: Use two regex.

19. Practice #2
$last_name =~ s{^s+}{};
$last_name =~ s{s+$}{};

20. Non-destructive
Substitution
$full_name = "betsy ross";
$new_name = $full_name =~ s{s+}{-}r; # r modifier
print "$full_name becomes $new_namen";

21. Character Symbols
.
s
S
d
D
.
Any Character
White space
Non-whitespace
Digit, 0-9
Non digit
Back slash
Literal Dot
Quote meta-character

22. Negated match
$last_name !~ m{^r.*o.*ss$};

23. Practice #3
Write a regex that determines if a non-empty string is entirely
composed of digits.
Bonus: do it two different ways

24. Practice #3
$str =~ m{^d+$};
$str !~ m{D};

25. Character class
split m{[.,;s]+}, $names;
$names =~ s{[^a-zA-Z0-9]}{-}g;
# leading ^ will negate the character class
# g modifier for global
# run character_class.pl

26. Modifiers
r
i
g
Non-destructive substitution
Case insensitive
Global match

27. Case insensitive
$last_name =~ m{^r.*o.*ss$}i;

28. Global match
@list = $str =~ m{[a-z]+}g;
$scalar = $str =~ m{[a-z]+}g;
$str =~ s{[^a-z]+}{-}g;

29. Grouping
($prefix, $last_name)
= $str =~ m{(mr[s]?[.]?)s+(S+)}i;

30. Alternation
($prefix, $last_name) = $str
=~ m{(mister|misses|miss|mr[s]?[.]?)s+(S+)}i;

31. Alternation
non-capturing group
($prefix, $last_name) = $str
=~ m{(mister|miss(?:es)?|mr[s]?[.]?)s+(S+)}i;
# the 'es' is in a non-capturing group

32. Alternation
non-capturing group
($prefix, $last_name) = $str
=~ m{(mi(?:ster|ss(?:es)?)|mr[s]?[.]?)s+(S+)}i;
# Too much non-capturing groups. Hard to read

33. Modifiers
r
i
g
n
Non-destructive substitution
Case insensitive
Global match
Make all groups, non-capturing

34. Practice #4
# what value is printed for year?
my $str = "Copyright 2013";
my $year;
($year) = $str =~ m{.*([0-9]+)};
print "$yearn";

35. Practice #4
Year is 3

36. Greedy vs. non-greedy
($year) = $str =~ m{.*([0-9]+)};
# $year is 3
# greedy maximizes the matching
($year) = $str =~ m{.*?([0-9]+)};
# $year is 2013
# non-greedy minimizes the matching

37. Non-greedy
*?
+?
??

38. Back reference
my $str = "****Spangled****";
my ($star, $word) = $str =~ m{^([*]+)([^*]+)1$};

39. Match Variables
$str = "Betsy Ross";
$str =~ s{(S+)s(S+)}{$2, $1};
print "$strn";
print "1: $1n";
print "2: $2n";

40. Practice #5
Using regex, remove all spaces from the beginning and end of a
string. This time, do it with one regex, not two.

41. Practice #5
$str =~ s{^s*(.+?)s*$}{$1};

42. Look-around
?<=
?=
Look behind
Look ahead

43. Look-around
my $pop = 281421906;
# 281,421,906
print "The US population is $popn";

44. Look-around
my $pop = 281421906;
# 281,421,906
print "The US population is $popn";
# add commas inbetween each three digits
$pop =~ s{(?<=d)(?=(ddd)+$)}{,}g;
print "The US population is $popn";

45. Negated Look-around
?<!
?!
Negated look behind
Negated look ahead

46. /xms
/x whitespace and comments
/m awk/grep/sed matching ^ and $
/s multi-line text (‘.’ matches newline)

47. A and z
# by default, beginning and end of string
# with m, beginning and end of line
$last_name =~ m{^r.*o.*ss$};
# always beginning and end of string
$last_name =~ m{Ar.*o.*ssz};

48. Anchors
^
$
A
z
b
Begins with
Ends with
String begins with
String ends with
Word boundary

49. tr
my $name = "JagerMech";
$name =~ tr/A-Z/a-z/;
# one for one translation.
# replace chars of left with those on right.
print "$namen";
# jagermech
# tr is almost never used

50. w
Not used on the homework.
Almost never used.
w describes alphanumerics and ‘_’

51. Further Reading
Mastering Regular Expressions
perldoc perlrequick
perldoc perlretut
perldoc perlre