Introduction to perl programming:
the minimum to know!
Fredj Tekaia
Institut Pasteur
tekaia@pasteur.fr
Bioinformatic and Comparative Genome Analysis Course
HKU-Pasteur Research Centre - Hong Kong, China
August 17 - August 29, 2009
perl
A basic program
#!/bin/perl
# Program to print a message
print 'Hello world.';
# Print a message
Variables, Arrays
$val=9;
$val=“9”;
$val=“ABC transporter”;
• case sensitive: $val is different from $Val
Operations and Assignment
Perl uses arithmetic operators:
$a = 1 + 2;
# Add 1 and 2 and store in $a
$a = 3 - 4;
# Subtract 4 from 3 and store in $a
$a = 5 * 6;
# Multiply 5 and 6
$a = 7 / 8;
# Divide 7 by 8 to give 0.875
$a = 9 ** 10; # Nine to the power of 10
$a = 5 % 2;
# Remainder of 5 divided by 2
$a++;
# Return $a and then increment it
$a--;
# Return $a and then decrement it
for strings perl has among others:
$a = $b . $c; # Concatenate $b and $c
$a = $b x $c; # $b repeated $c times
To assign values perl includes
$a
$a
$a
$a
= $b;
+= $b;
-= $b;
.= $b;
#
#
#
#
Assign $b to $a
Add $b to $a
Subtract $b from $a
Append $b onto $a
Array variables
An array variable is a list of scalars (ie
numbers and/or strings).
they are prefixed by: @
@SEQNAME
= (”MG001", ”MG002", ”MG003");
$SEQNAME[2]
(MG003)
Attention: 0, 1, 2,....
@num = (0,1,2,3);
@L_CODONS = ('TTT','TTC','TTA','TTG',
'CTT','CTC','CTA','CTG',
'ATT','ATC','ATA','ATG',
'GTT','GTC','GTA','GTG',
'TCT','TCC','TCA','TCG',
'CCT','CCC','CCA','CCG',
'ACT','ACC','ACA','ACG',
'GCT','GCC','GCA','GCG',
'TAT','TAC','TAA','TAG',
'CAT','CAC','CAA','CAG',
'AAT','AAC','AAA','AAG',
'GAT','GAC','GAA','GAG',
'TGT','TGC','TGA','TGG',
'CGT','CGC','CGA','CGG',
'AGT','AGC','AGA','AGG',
'GGT','GGC','GGA','GGG');
@AA = ('A','R','N','D','C','Q','E','G','H','I','L','K','M','F','P','S','T','W','Y','V','B');
@mm = ( 'a','r','n','d','c','q','e','g','h','i','l','k','m','f','p','s','t','w','y','v','b’ );
Associative arrays : hash tables
Ordinary list arrays allow us to access their
element by number. The first element of array
@AA is $AA[0]. The second element is $AA[1],
and so on.
But perl also allows us to create arrays which
are accessed by string. These are called
associative arrays.
array itself is prefixed by a % sign
%ages = (”Michael", 39,
"Angie", 27,
"Willy", "21 years",
"The Queen Mother", 108);
$ages{"Michael"};
# Returns 39
$ages{"Angie"};
# Returns 27
$ages{"Willy"};
# Returns "21 years"
$ages{"The Queen Mother"};
# Returns 108
File handling
a script (cat.pl) equivalent to the UNIX cat:
#!/bin/perl
open(FILE,”GMG.pep”);
while <FILE>
{
print $_;
}
close (FILE);
use: chmod a+x cat.pl ; cat.pl
split
A very useful function in perl: splits up a string and places it into an
array.
#!/bin/perl
open(FILE,”GMG.pep”);
while <FILE>
{
@tab=split(/\s+/, $_);
print $tab[0];
}
close (FILE);
#!/bin/perl
open(FILE,”GMG.pep”);
while <FILE>
{
@tab=split(/\s+/, $_, 2);
$NOM{$tab[0]} = $tab[1];
print $NOM{$tab[0]} ;
}
close (FILE);
@tab=split(/\s+/,$_,n);
Control structures
foreach
To go through each line of an array or other
list-like structure (such as lines in a file)
perl uses the foreach structure. This has the
form
foreach $nom (@SEQNAME) # Visit each item in
# and call it $nom
{
print "$nom\n";# Print the item
}
turn
foreach $j ( 0 .. 2)#
Visit each value in turn
# and call it $j
{
print "$SEQNAM[$j]\n";# Print the item
}
foreach $j ( 0 .. $#AA) # Visit each value
# and call it $j
{
print "$AA[$j]\n";# Print the item
}
in turn
Testing
Here are some tests on numbers and strings.
$a == $b
$a != $b
$a eq $b
$a ne $b
# Is $a numerically equal to $b?
#Beware: Don't use the = operator.
# Is $a numerically unequal to $b?
# Is $a string-equal to $b?
# Is $a string-unequal to $b?
You can also use logical and, or and not:
($a && $b)
($a || $b)
!($a)
# Is $a and $b true?
# Is either $a or $b true?
# is $a false?
for
for (initialise; test; inc)
{
first_action;
second_action;
etc....
}
for ($i = 0; $i < 10; ++$i)
# Start with $i = 1
# Do it while $i < 10
#Increment $i before repeating
{
print "$i\n";
}
Conditionals
if ($a)
{
print "The string is not empty\n";
}
else
{
print "The string is empty\n";
}
#!/bin/perl
open(FILE,”GMG.pep”);
while <FILE>
{
print $_ if ( m/>/ );
}
close (FILE);
String matching
$a eq $b
$a ne $b
# Is $a string-equal to $b?
# Is $a string-unequal to $b?
Here are some special RE characters and their
meaning
.
^
$
*
+
?
#
#
#
#
#
#
Any single character except a newline
The beginning of the line or string
The end of the line or string
Zero or more of the last character
One or more of the last character
Zero or one of the last character
Some more special characters
\n
\t
\w
\W
\d
\D
\s
\S
\b
\B
#
#
#
#
#
#
#
#
#
#
#
#
#
A newline
A tab
Any alphanumeric (word) character.
The same as [a-zA-Z0-9_]
Any non-word character.
The same as [^a-zA-Z0-9_]
Any digit. The same as [0-9]
Any non-digit. The same as [^0-9]
Any whitespace character: space,
tab, newline, etc
Any non-whitespace character
A word boundary, outside [] only
No word boundary
Characters like $,
peculiar cases in
want to match for
to preceed it by a
\|
\[
\)
\*
\^
\/
\\
#
#
#
#
#
#
#
|, [, ), \, / and so on are
regular expressions. If you
one of those then you have
backslash (\). So:
Vertical bar
An open square bracket
A closing parenthesis
An asterisk
A carat symbol
A slash
A backslash
Substitution and translation
s/london/London/
$sentence =~ s/london/London/
global substitution; i option (for "ignore case").
s/london/London/gi
Translation
$sentence =~ tr/abc/edf/
tr/a-z/A-Z/;
#converts $_ to upper case
tr/A-Z/a-z/;
#converts $_ to lower case