Regular Expression Learning Objectives: 1. To understand the concept of regular expression 2. To learn commonly used operations involving regular expression

Regular Expression

Learning Objectives:1. To understand the concept of regular

expression

2. To learn commonly used operations involving regular expression / pattern matching

3. To learn the special cases occurred in regular expression / pattern matching

COMP111COMP111Lecture 16 / Slide Lecture 16 / Slide 22

Regular ExpressionTable 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


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.

What is a Regular Expression?


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.

Simple Uses of Regular Expressions (1)


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.



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(<>){if(/Shakespeare/){

print;}

}



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”.



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.*



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.

Substitution (1)


The program:$ cat movieTitanicSaving Private RyanShakespeare in LoveLife is Beautiful$ cat sub1#!/usr/local/bin/perl5 -wwhile(<>){

if(/Shakespeare/){s/Shakespeare/Bill Gates/;print;

}}$ sub1 movieBill Gates in Love$

Substitution (2)


An even shorter way to write it:

$ cat sub2#!/usr/local/bin/perl5 -wwhile(<>){

if(s/Shakespeare/Bill Gates/){print;

}}$ sub2 movieBill Gates in Love$

Substitution (3)


A regular expression is a pattern.

Some parts of the pattern match a single character (a).

Other parts of the pattern match multiple characters (.*).

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 testhihi bob.$ cat sub3 test#!/usr/local/bin/perl5 -wwhile(<>){

if(/\./){ print; }}$ sub3 testhi bob.$

Single-Character Patterns


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.

Single-Character Groups (1)


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):/[0123456789]/ # 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



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.

/[^0123456789]/ # match any single non-digit/[^0-9]/ # same

/[âeiouAEIOU]/ # match any single non-vowel/[^\^]/ # match any single character except ^



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) [â-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



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).

Multipliers (1)


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

Multipliers (2)


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}/

General Multiplier (1)


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}

General Multiplier (2)


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/

Pattern Memory (1)


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)

Pattern Memory (2)


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-.


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.


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/.


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 $


So what happens with the pattern: a|b* Is this (a|b)* or

a|(b*) ? Precedence of patterns from highest to lowest:

Name RepresentationParentheses ( )Multipliers ? + * {m,n}Sequence & anchoring abc ^ $Alternation |

By the table, * has higher precedence than |, so it is interpreted as a|(b*).

Precedence (1)


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).


Precedence (3)

More precedence examples:abc* # matches ab, abc, abcc, abccc,…(abc)* # matches "", abc, abcabc, abcabcabc,…â|b # matches a at beginning of line, or b anywhere^(a|b) # matches either a or b at the beginning of linea|bc|d # a, or bc, or d(a|b)(c|d) # ac, ad, bc, or bd(Bill Gates)|(Bill Clinton) # Bill Gates, Bill ClintonBill (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


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";

}


Using =~ (2)

An example using =~ to match <STDIN>:$ cat match1#!/usr/local/bin/perl5 -wprint "Quit (y/n)? ";if(<STDIN> =~ /^[yY]/){

print "Quitting\n";exit;

}print "Continuing\n";$ match1Quit (y/n)? yQuitting$


Another example using =~ to match <STDIN>:$ cat match2#!/usr/local/bin/perl5 -wprint "Wakeup (y/n)? ";while(<STDIN> =~ /^[nN]/){

print "Sleeping\n";print "Wakeup (y/n)? ";

}$ match2Wakeup (y/n)? nSleepingWakeup (y/n)? NSleepingWakeup (y/n)? y$

Using =~ (3)


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 -wprint "Quit (y/n)? ";if(<STDIN> =~ /^y/i){

print "Quitting\n";exit;

}print "Continuing\n";$ match1aQuit (y/n)? YQuitting$


Slash and Backslash

If your pattern has a slash character (/), you must precede each with a backslash (\):

$ cat slash1#!/usr/local/bin/perl5 -wprint "Enter path: ";$path = <STDIN>;if($path =~ /^\/usr\/local\/bin/){

print "Path is /usr/local/bin\n";}$ slash1Enter path: /usr/local/binPath is /usr/local/bin$


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 -wprint "Enter path: ";$path = <STDIN>;if($path =~ m#^/usr/local/bin#){

# if($path =~ m@^/usr/local/bin@){ # also worksprint "Path is /usr/local/bin\n";

}$ slash1aEnter path: /usr/local/binPath is /usr/local/bin$

Different Pattern Delimiters


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 $1\n";$ read1The first name of Shakespeare is Bill

Special Read-Only Variables (1)


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



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: $'\n";$ read3Before: Bill ShakespeareMatch: in After: Love



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: $_\n";$_ = "Bill Shakespeare in love with Bill Gates";s/Bill/William/g; print "Sub2: $_\n";$ sub3Sub1: William Shakespeare in love with Bill GatesSub2: William Shakespeare in love with William Gates$

More on Substitution (1)


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";$ sub4William Shakespeare in love with William Gates$



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>$



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; $line =~ s@Shakespeare@Gates@gi; print "$line\n";$ sub6William Gates in love with William Gates$



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 $line using space as delimiterprint "$fields[0] $fields[3] $fields[6]\n";$ split1Bill love Gates$

Split (1)


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 $line using space (default) as delimiterprint "$fields[0] $fields[3] $fields[6]\n";$ split2Bill love Gates$

Split (2)


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);$line = join(" ", @list);print "$line\n";$ join1Bill Shakespeare dislikes Bill Gates $

Note that the glue string is not a regular expression, just an normal string.

join

Documents

Regular Expression Learning Objectives: 1. To understand the concept of regular expression 2. To learn commonly used operations involving regular expression