COMP284 Scripting Languages – Handouts
COMP284 Scripting Languages
Lecture 4: Perl (Part 3)
Handouts
Ullrich Hustadt
Department of Computer Science
School of Electrical Engineering, Electronics, and Computer Science
University of Liverpool
Contents
1 Regular expressions (1)
Introduction
Characters
Character classes
Quantifiers
COMP284 Scripting Languages Lecture 4 Slide L4 – 1
Regular expressions (1) Introduction
Regular expressions: Motivation
Suppose you are testing the performance of a new sorting algorithm by
measuring its runtime on randomly generated arrays of numbers
of a given length:
Generating an unsorted array with 10000 elements took 1.250 seconds
Sorting took 7.220 seconds
Generating an unsorted array with 10000 elements took 1.243 seconds
Sorting took 10.486 seconds
Generating an unsorted array with 10000 elements took 1.216 seconds
Sorting took 8.951 seconds
Your task is to write a program that determines the average runtime of
the sorting algorithm:
Average runtime for 10000 elements is 8.886 seconds
Solution: The regular expression /^Sorting took (\d+\.\d+) seconds/
allows us to get the required information
; Regular expressions are useful for information extraction
COMP284 Scripting Languages Lecture 4 Slide L4 – 2
Regular expressions (1) Introduction
Regular expressions: Motivation
Suppose you have recently taken over responsibility for a company’s
website. You note that their HTML files contain a large number of
URLs containing superfluous occurrences of ‘..’, e.g.
http://www.myorg.co.uk/info/refund/../vat.html
Your task is to write a program that replaces URLs like these with
equivalent ones without occurrences of ‘..’:
http://www.myorg.co.uk/info/vat.html
while making sure that relative URLs like
../video/disk.html
are preserved
Solution: s!/[^\/]+/\.\.!!; removes a superfluous dot-segment
; Substitution of regular expressions is useful for text manipulation
COMP284 Scripting Languages Lecture 4 Slide L4 – 3
Regular expressions (1) Introduction
Regular expressions: Introductory example
\Ahttps ?:\/\/[^\/]+\/.\w.\/( cat|dog )\/\1
• \A is an assertion or anchor
• h, t, p, s, :, \/, c, a, t, d, o, g are characters
• ? and + are quantifiers
• [^\/] is a character class
• . is a metacharacter and \w is a special escape
• (cat|dog) is alternation within a capture group
• \1 is a backreference to a capture group
COMP284 Scripting Languages Lecture 4 Slide L4 – 4
Regular expressions (1) Introduction
Pattern match operation
• To match a regular expession regexpr against the special variable $_
simply use one of the expressions /regexpr/ or m/regexpr/
• This is called a pattern match
• $_ is the target string of the pattern match
• In a scalar context a pattern match returns true (1) or false (’’)
depending on whether regexpr matches the target string
if (/\ Ahttps ?:\/\/[^\/]+\/.\w.\/( cat|dog )\/\1/) {
… }
if (m/\ Ahttps ?:\/\/[^\/]+\/.\w.\/( cat|dog )\/\1/) {
… }
COMP284 Scripting Languages Lecture 4 Slide L4 – 5
Regular expressions (1) Characters
Regular expressions: Characters
The simplest regular expression just consists of a sequence of
• alphanumberic characters and
• non-alphanumeric characters escaped by a backslash:
that matches exactly this sequence of characters occurring as a substring
in the target string
$_ = “ababcbcdcde”;
if (/cbc/) { print “Match\n”} else { print “No match\n” }
Output:
Match
$_ = “ababcbcdcde”;
if (/dbd/) { print “Match\n”} else { print “No match\n” }
Output:
No match
COMP284 Scripting Languages Lecture 4 Slide L4 – 6
Regular expressions (1) Characters
Regular expressions: Special variables
• Often we do not just want to know whether a regular expession matches
a target string, but retrieve additional information
• The special variable $-[0] can be used to retrieve the start position of
the match
Note that positions in strings are counted starting with 0
• The special variable $+[0] can be used to retrieve the first position
after the match
• The special variable $& returns the match itself
$_ = “ababcbcdcde”;
if (/cbc/) { print “Match found at position $-[0]: $&\n”}
Output:
Match found at position 4: cbc
COMP284 Scripting Languages Lecture 4 Slide L4 – 7
Regular expressions (1) Characters
Regular expressions: Special escapes
There are various special escapes and metacharacters that match more
then one character:
. Matches any character except \n
\w Matches a ‘word’ character (alphanumeric
plus ‘_’, plus other connector punctuation
characters plus Unicode characters
\W Matches a non-‘word’ character
\s Match a whitespace character
\S Match a non-whitespace character
\d Match a decimal digit character
\D Match a non-digit character
\p{UnicodeProperty} Match UnicodeProperty characters
\P{UnicodeProperty} Match non-UnicodeProperty characters
COMP284 Scripting Languages Lecture 4 Slide L4 – 8
Regular expressions (1) Characters
Regular expressions: Unicode properties
• Each unicode character has one or more properties,
for example, which script it belongs it
• \p{UnicodeProperty} matches all characters that have a particular
property
• \P{UnicodeProperty} matches those that do not
• Examples of unicode properties are
Arabic Arabic characters
ASCII ASCII characters
Currency_Symbol Currency symbols
Digit Digits in all scripts
Greek Greek characters
Han Chinese kanxi or Japanese kanji characters
Space Whitespace characters
See http://perldoc.perl.org/perluniprops.html for a complete list
COMP284 Scripting Languages Lecture 4 Slide L4 – 9
http://perldoc.perl.org/perluniprops.html
Regular expressions (1) Character classes
Regular expressions: Character class
• A character class, a list of characters, special escapes, metacharacters
and unicode properties enclosed in square brackets, matches any single
character from within the class,
for example, [ad\t\n\-\\09]
• One may specify a range of characters with a hyphen -,
for example, [b-u]
• A caret ^ at the start of a character class negates/complements it,
that is, it matches any single character that is not from within the class,
for example, [^01a-z]
$_ = “ababcbcdcde”;
if (/[bc][b-e][^ bcd ]/) {
print “Match at positions $-[0] to “,$+[0]-1,”: $&\n”};
Output:
Match at positions 8 to 10: cde
COMP284 Scripting Languages Lecture 4 Slide L4 – 10
Regular expressions (1) Quantifiers
Quantifiers
• The constructs for regular expressions that we have so far are not
sufficient to match, for example, natural numbers of arbitrary size
• Also, writing a regular expressions for, say, a nine digit number
would be tedious
This is made possible with the use of quantifiers
regexpr* Match regexpr 0 or more times
regexpr+ Match regexpr 1 or more times
regexpr? Match regexpr 1 or 0 times
regexpr{n} Match regexpr exactly n times
regexpr{n,} Match regexpr at least n times
regexpr{n,m} Match regexpr at least n but not more than m times
Quantifiers are greedy by default and match the longest leftmost sequence
of characters possible
COMP284 Scripting Languages Lecture 4 Slide L4 – 11
Regular expressions (1) Quantifiers
Quantifiers
regexpr* Match regexpr 0 or more times
regexpr+ Match regexpr 1 or more times
regexpr? Match regexpr 1 or 0 times
regexpr{n} Match regexpr exactly n times
regexpr{n,} Match regexpr at least n times
regexpr{n,m} Match regexpr at least n but not more than m times
Example:
$_ = “Sorting took 10.486 seconds”;
if (/\d+\.\d+/) {
print “Match at positions $-[0] to “,$+[0]-1,”: $&\n”};
$_ = “E00481370”;
if (/[A-Z]0{2}(\d+)/) {
print “Match at positions $-[1] to “,$+[1]-1,”: $1\n”};
Output:
Match at positions 13 to 18: 10.486
Match at positions 3 to 8: 481370
COMP284 Scripting Languages Lecture 4 Slide L4 – 12
Regular expressions (1) Quantifiers
Quantifiers
Example:
$_ = “E00481370”;
if (/\d+/) {
print “Match at positions $-[0] to “,$+[0]-1,”: $&\n”};
Output:
Match at positions 1 to 8: 00481370
• The regular expression \d+ matches 1 or more digits
• As the example illustrates, the regular expression \d+
• matches as early as possible
• matches as many digits as possible
; quantifiers are greedy by default
COMP284 Scripting Languages Lecture 4 Slide L4 – 13
Regular expressions (1) Quantifiers
Revision
Read
• Chapter 7: In the World of Regular Expressions
• Chapter 8: Matching with Regular Expressions
of
R. L. Schwartz, brian d foy, T. Phoenix:
Learning Perl.
O’Reilly, 2011.
• http://perldoc.perl.org/perlre.html
• http://perldoc.perl.org/perlretut.html
• http://www.perlfect.com/articles/regextutor.shtml
COMP284 Scripting Languages Lecture 4 Slide L4 – 14
http://perldoc.perl.org/perlre.html
http://perldoc.perl.org/perlretut.html
http://www.perlfect.com/articles/regextutor.shtml
Lecture 4
Regular expressions (1)
Introduction
Characters
Character classes
Quantifiers