1. Regular Expression Learning Objectives:
To understand the concept of regular expression
To learn commonly used operations involving regular expression / pattern matching
To learn the special cases occurred in regular expression / pattern matching

2. Regular Expression Table of Content What is a Regular Expression ?
Simple Uses of Regular Expressions
Substitution
Patterns
Single-Character Patterns
Single-Character Groups
Multipliers
General Multiplier
Pattern Memory
Alteration
Anchoring Patterns
Precedence
Using =~
Ignoring Case
Slash and Backslash
Different Pattern Delimiters
Special Read-Only Variables
More on Substitution
split
join

3. What is a Regular Expression?
A regular expression is a pattern to be matched against a string. For example, the pattern Bill.
Matching either succeeds or fails.
Sometimes you may want to replace a matched pattern with another string.
Regular expressions are used by many other Unix commands and programs, such as grep, sed, awk, vi, emacs, and even some shells.

4. Simple Uses of Regular Expressions (1)
If we are looking for all the lines in a file that contain the string Shakespeare, we could use the grep command:
$ grep Shakespeare movie > result
Here, Shakespeare is the regular expression that grep looks for in the file movie.
Lines that match are redirected to result.

5. Simple Uses of Regular Expressions (2)
In Perl, we can make Shakespeare a regular expression by enclosing it in slashes:
if(/Shakespeare/){
print $_; }
What is tested in the if-statement?
Answer: $_.
When a regular expression is enclosed in slashes, $_ is tested against the regular expression, returning true if there is a match, false otherwise.

6. Simple Uses of Regular Expressions (3)
if(/Shakespeare/){
print $_; }
The previous example tests only one line, and prints out the line if it contains Shakespeare.
To work on all lines, add a loop:
while(<>){
print;

7. Simple Uses of Regular Expressions (4)
What if we are not sure how to spell Shakespeare?
Certainly the first part is easy Shak, and there must be a r near the end.
How can we express our idea?
grep: grep "Shak.*r" movie > result
Perl: while(<>){
if(/Shak.*r/){
print; }
.* means “zero or more of any character”.

8. Simple Uses of Regular Expressions (5)
grep: grep "Shak.*r" movie > result
The double quotes in this grep example are needed to prevent the shell from interpreting * as “all files”.
Since Shakespeare ends in “e”, shouldn’t it be:
Shak.*r.*
Answer: No need. Any character can come before or after the pattern.
Shak.*r is the same as .*Shak.*r.*

9. Substitution (1)
Another simple regular expression is the substitute operator.
It replaces part of a string that matches the regular expression with another string.
s/Shakespeare/Bill Gates/;
$_ is matched against the regular expression (Shakespeare).
If the match is successful, the part of the string that matched is discarded and replaced by the replacement string (Bill Gates).
If the match is unsuccessful, nothing happens.

10. Substitution (2) The program: $ cat movie Titanic Saving Private Ryan
Shakespeare in Love
Life is Beautiful
$ cat sub1
#!/usr/local/bin/perl5 -w
while(<>){
if(/Shakespeare/){
s/Shakespeare/Bill Gates/;
print; }
$ sub1 movie
Bill Gates in Love $

11. Substitution (3) An even shorter way to write it: $ cat sub2
#!/usr/local/bin/perl5 -w
while(<>){
if(s/Shakespeare/Bill Gates/){
print; }
$ sub2 movie
Bill Gates in Love $

12. Patterns A regular expression is a pattern.
Some parts of the pattern match a single character (a).
Other parts of the pattern match multiple characters (.*).

13. Single-Character Patterns
The dot “.” matches any single character except the newline (\n).
For example, the pattern /a./ matches any two-letter sequence that starts with a and is not “a\n”.
Use \. if you really want to match the period.
$ cat test hi
hi bob.
$ cat sub3 test
#!/usr/local/bin/perl5 -w
while(<>){
if(/\./){ print; } }
$ sub3 test $

14. Single-Character Groups (1)
If you want to specify one out of a group of characters to match use [ ]:
/[abcde]/
This matches a string containing any one of the first 5 lowercase letters, while:
/[aeiouAEIOU]/
matches any of the 5 vowels in either upper or lower case.

15. Single-Character Groups (2)
If you want ] in the group, put a backslash before it, or put it as the first character in the list:
/[abcde]]/ # matches [abcde] + ]
/[abcde\]]/ # okay
/[]abcde]/ # also okay
Use - for ranges of characters (like a through z):
/[ ]/ # any single digit /[0-9]/ # same
If you want - in the list, put a backslash before it, or put it at the beginning/end:
/[X-Z]/ # matches X, Y, Z
/[X\-Z]/ # matches X, -, Z
/[XZ-]/ # matches X, Z, -
/[-XZ]/ # matches -, X, Z

16. Single-Character Groups (3)
More range examples:
/[0-9\-]/ # match 0-9, or minus
/[0-9a-z]/ # match any digit or lowercase letter
/[a-zA-Z0-9_]/ # match any letter, digit, underscore
There is also a negated character group, which starts with a ^ immediately after the left bracket. This matches any single character not in the list.
/[^ ]/ # match any single non-digit /[^0-9]/ # same
/[^aeiouAEIOU]/ # match any single non-vowel
/[^\^]/ # match any single character except ^

17. Single-Character Groups (4)
For convenience, some common character groups are predefined:
Predefined Group Negated Negated Group
\d (a digit) [0-9] \D (non-digit) [^0-9]
\w (word char) [a-zA-Z0-9_] \W (non-word) [^a-zA-Z0-9_]
\s (space char) [ \t\n] \S (non-space) [^ \t\n]
\d matches any digit
\w matches any letter, digit, underscore
\s matches any space, tab, newline
You can use these predefined groups in other groups:
/\da-fA-F/ # match any hexadecimal digit

18. Multipliers (1)
Multipliers allows you to say “one or more of these” or “up to four” of these.”
* means zero or more of the immediately previous character (or character group).
+ means one or more of the immediately previous character (or character group).
? means zero or one of the immediately previous character (or character group).

19. Multipliers (2) Example:
/Ga+te?s/
matches a G followed by one or more a’s followed by t, followed by an optional e, followed by s.
*, +, and ? are greedy, and will match as many characters as possible: $_ = "Bill xxxxxxxxx Gates";
s/x+/Cheap/; # gives: Bill Cheap Gates

20. General Multiplier (1) How do you say “five to ten x’s”?
/xxxxxx?x?x?x?x?/ # works, but ugly
/x{5,10}/ # nicer
How do you say “five or more x’s”?
/x{5,}/
How do you say “exactly five x’s”?
/x{5}/
How do you say “up to five x’s”?
/x{0,5}/

21. General Multiplier (2)
How do you say “c followed by any 5 characters (which can be different) and ending with d”?
/c.{5}d/
* is the same as {0,}
+ is the same as {1,}
? is the same as {0,1}

22. Pattern Memory (1)
How would we match a pattern that starts and ends with the same letter or word?
For this, we need to remember the pattern.
Use ( ) around any pattern to put that part of the string into memory (it has no effect on the pattern itself).
To recall memory, include a backslash followed by an integer.
/Bill(.)Gates\1/

23. Pattern Memory (2) Example:
/Bill(.)Gates\1/
This example matches a string starting with Bill, followed by any single non-newline character, followed by Gates, followed by that same single character.
So, it matches:
Bill!Gates! Bill-Gates-
but not:
Bill?Gates! Bill-Gates_
(Note that /Bill.Gates./ would match all four)

24. Pattern Memory (3) More examples:
/a(.)b(.)c\2d\1/
This example matches a string starting with a, a character (#1), followed by b, another single character (#2), c, the character #2, d, and the character #1.
So it matches: a-b!c!d-.

25. Pattern Memory (4)
The reference part can have more than a single character.
For example:
/a(.*)b\1c/
This example matches an a, followed by any number of characters (even zero), followed by b, followed by the same sequence of characters, followed by c.
So it matches: aBillbBillc and abc, but not: aBillbBillGatesc.

26. Alteration
How about picking from a set of alternatives when there is more than one character in the patterns.
The following example matches either Gates or Clinton or Shakespeare:
/Gates|Clinton|Shakespeare/
For single character alternatives,
/[abc]/
is the same as
/a|b|c/.

27. Anchoring Patterns
Anchors requires that the pattern be at the beginning or end of the line.
^ matches the beginning of the line (only if ^ is the first character of the pattern):
/^Bill/ # match lines that begin with Bill
/^Gates/ # match lines that begin with Gates
/Bill\^/ # match lines containing Bill^ somewhere
/\^/ # match lines containing ^
$ matches the end of the line (only if $ is the last character of the pattern):
/Bill$/ # match lines that end with Bill
/Gates$/ # match lines that end with Gates
/$Bill/ # match with contents of scalar $Bill
/\$/ # match lines containing $

28. Precedence (1) So what happens with the pattern: a|b*
Is this (a|b)* or
a|(b*) ?
Precedence of patterns from highest to lowest:
Name Representation
Parentheses ( )
Multipliers ? + * {m,n}
Sequence & anchoring abc ^ $
Alternation |
By the table, * has higher precedence than |, so it is interpreted as a|(b*).

29. Precedence (2)
What if we want the other interpretation in the previous example?
Answer: Simple, just use parentheses: (a|b)*
Use parentheses in ambiguous cases to improve clarity, even if not strictly needed.
When you use parentheses for precedence, they also go into memory (\1, \2, \3).

30. Precedence (3) More precedence examples:
abc* # matches ab, abc, abcc, abccc,…
(abc)* # matches "", abc, abcabc, abcabcabc,…
^a|b # matches a at beginning of line, or b anywhere
^(a|b) # matches either a or b at the beginning of line
a|bc|d # a, or bc, or d
(a|b)(c|d) # ac, ad, bc, or bd
(Bill Gates)|(Bill Clinton) # Bill Gates, Bill Clinton
Bill (Gates|Clinton) # Bill Gates, Bill Clinton
(Mr\. Bill)|(Bill (Gates|Clinton))
# Mr. Bill, Bill Gates, Bill Clinton
(Mr\. )?Bill( Gates| Clinton)?
# Bill, Mr. Bill, Bill Gates, Bill Clinton,
# Mr. Bill Gates, Mr. Bill Clinton

31. Using =~ (1) What if you want to match a different variable than $_?
Answer: Use =~.
Examples:
$name = "Bill Shakespeare";
$name =~ /^Bill/; # true
$name =~ /(.)\1/; # also true (matches ll)
if($name =~ /(.)\1/){
print "$name\n"; }

32. Using =~ (2) An example using =~ to match <STDIN>: $ cat match1
#!/usr/local/bin/perl5 -w
print "Quit (y/n)? ";
if(<STDIN> =~ /^[yY]/){
print "Quitting\n";
exit; }
print "Continuing\n";
$ match1
Quit (y/n)? y
Quitting $

33. Using =~ (3) Another example using =~ to match <STDIN>:
$ cat match2
#!/usr/local/bin/perl5 -w
print "Wakeup (y/n)? ";
while(<STDIN> =~ /^[nN]/){
print "Sleeping\n"; }
$ match2
Wakeup (y/n)? n
Sleeping
Wakeup (y/n)? N
Wakeup (y/n)? y $

34. Ignoring Case
In the previous examples, we used [yY] and [nN] to match either upper or lower case.
Perl has an “ignore case” option for pattern matching: /somepattern/i
$ cat match1a
#!/usr/local/bin/perl5 -w
print "Quit (y/n)? ";
if(<STDIN> =~ /^y/i){
print "Quitting\n";
exit; }
print "Continuing\n";
$ match1a
Quit (y/n)? Y
Quitting $

35. Slash and Backslash
If your pattern has a slash character (/), you must precede each with a backslash (\):
$ cat slash1
#!/usr/local/bin/perl5 -w
print "Enter path: ";
$path = <STDIN>;
if($path =~ /^\/usr\/local\/bin/){
print "Path is /usr/local/bin\n"; }
$ slash1
Enter path: /usr/local/bin
Path is /usr/local/bin $

36. Different Pattern Delimiters
If your pattern has lots of slash characters (/), you can also use a different pattern delimiter with the form: m#somepattern#
The # can be any non-alphanumeric character.
$ cat slash1a
#!/usr/local/bin/perl5 -w
print "Enter path: ";
$path = <STDIN>;
if($path =~ m#^/usr/local/bin#){
# if($path =~ # also works
print "Path is /usr/local/bin\n"; }
$ slash1a
Enter path: /usr/local/bin
Path is /usr/local/bin $

37. Special Read-Only Variables (1)
After a successful pattern match, the variables $1, $2, $3,… are set to the same values as \1, \2, \3,…
You can use $1, $2, $3,… later in your program.
$ cat read1
#!/usr/local/bin/perl5 -w
$_ = "Bill Shakespeare in Love";
/(\w+)\W+(\w+)/; # match first two words
# $1 is now "Bill" and $2 is now "Shakespeare"
print "The first name of $2 is $1\n";
$ read1
The first name of Shakespeare is Bill

38. Special Read-Only Variables (2)
You can also use $1, $2, $3,… by placing the match in a list context:
$ cat read2
#!/usr/local/bin/perl5 -w
$_ = "Bill Shakespeare in Love";
($first, $last) = /(\w+)\W+(\w+)/;
print "The first name of $last is $first\n";
$ read2
The first name of Shakespeare is Bill

39. Special Read-Only Variables (3)
Other read-only variables:
$& is the part of the string that matched the pattern.
$` is the part of the string before the match
$’ is the part of the string after the match
$ cat read3
#!/usr/local/bin/perl5 -w
$_ = "Bill Shakespeare in Love";
/ in /;
print "Before: $`\n";
print "Match: $&\n";
print "After: $'\n";
$ read3
Before: Bill Shakespeare
Match: in
After: Love

40. More on Substitution (1)
If you want to replace all matches instead of just the first match, use the g option for substitution:
$ cat sub3
#!/usr/local/bin/perl5 -w
$_ = "Bill Shakespeare in love with Bill Gates";
s/Bill/William/;
print "Sub1: $_\n";
s/Bill/William/g;
print "Sub2: $_\n";
$ sub3
Sub1: William Shakespeare in love with Bill Gates
Sub2: William Shakespeare in love with William Gates $

41. More on Substitution (2)
You can use variable interpolation in substitutions:
$ cat sub4
#!/usr/local/bin/perl5 -w
$find = "Bill";
$replace = "William";
$_ = "Bill Shakespeare in love with Bill Gates";
s/$find/$replace/g;
print "$_\n";
$ sub4
William Shakespeare in love with William Gates $

42. More on Substitution (3)
Pattern characters in the regular expression allows patterns to be matched, not just fixed characters:
$ cat sub5
#!/usr/local/bin/perl5 -w
$_ = "Bill Shakespeare in love with Bill Gates";
s/(\w+)/<$1>/g;
print "$_\n";
$ sub5
<Bill> <Shakespeare> <in> <love> <with> <Bill> <Gates> $

43. More on Substitution (4)
Substitution also allows you to:
ignore case
use alternate delimiters
use =~
$ cat sub6
#!/usr/local/bin/perl5 -w
$line = "Bill Shakespeare in love with bill Gates";
$line =~ s#bill#William#gi;
$line =~
print "$line\n";
$ sub6
William Gates in love with William Gates $

44. Split (1) The split function allows you to break a string into fields.
split takes a regular expression and a string, and breaks up the line wherever the pattern occurs.
$ cat split1
#!/usr/local/bin/perl5 -w
$line = "Bill Shakespeare in love with Bill Gates";
@fields = split(/ /,$line);
# split $line using space as delimiter
print "$fields[0] $fields[3] $fields[6]\n";
$ split1
Bill love Gates $

45. Split (2) You can use $_ with split.
split defaults to look for space delimiters.
$ cat split2
#!/usr/local/bin/perl5 -w
$_ = "Bill Shakespeare in love with Bill Gates";
@fields = split;
# split $line using space (default) as delimiter
print "$fields[0] $fields[3] $fields[6]\n";
$ split2
Bill love Gates $

46. join The join function allows you to glue strings in a list together.
$ cat join1
#!/usr/local/bin/perl5 -w
@list = qw(Bill Shakespeare dislikes Bill Gates);
$line = join("
print "$line\n";
$ join1
Bill Shakespeare dislikes Bill Gates $
Note that the glue string is not a regular expression, just an normal string.