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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
Substitute operator / modifiers in Perl

Perl's substitute operator lets you replace a Regular Expression with another string within a target string. For example

$hello = "Grating";
$hello =~ s/a/ee/;
print "$hello\n";


Will turn Grating into Greeting within the $hello variable. You'll note that you can use almost any special character in place of the "/" delimiter, and that (unusually but not uniquely) the =~ operator in this use actually changes the content of the incoming variable.

There's a further "twist" to the substitute syntax too. What are those extra letters that you sometimes see at the end? If you add a "modifier" letter after the final delimiter, you can alter the behaviour of the whole substitution.

Modifier g is Global - replace ALL matches not just the first
modifier e is Execute - perform the output string as a piece of code
modifier i instruct Perl to ignore case in matching
modifier x tells Perl to treat spaces in the regular expression as comments
modifier s tells Perl to have . (full stop) match new line as well as anything else
modifier m tells Perl to have ^ and $ match at embedded new lines within the string.

Here's an example showing the syntax and effect of the e and g modifiers:

# Use of the s (substitute) operator
# Showing the "e" and "g" modifiers
 
$text = 'a little \piece of \bread and butter';
print "<b>Base text</b> $text\n";
 
$textcopy = $text;
$textcopy =~ s/(\s.)/uc($1)/;
print "<b>Single default substitute</b> $textcopy\n";
 
$textcopy = $text;
$textcopy =~ s/(\s.)/uc($1)/e;
print "<b>Substitute and execute</b> $textcopy\n";
 
$textcopy = $text;
$textcopy =~ s/(\s.)/uc($1)/g;
print "<b>Global Substitute</b> $textcopy\n";
 
$textcopy = $text;
$textcopy =~ s/(\s.)/uc($1)/eg;
print "<b>Global substitute and execute</b> $textcopy\n";



which generates

Base text a little \piece of \bread and butter
Single default substitute auc( l)ittle \piece of \bread and butter
Substitute and execute a Little \piece of \bread and butter
Global Substitute auc( l)ittleuc( \)pieceuc( o)fuc( \)breaduc( a)nduc( b)utter
Global substitute and execute a Little \piece Of \bread And Butter


My example uses ( .... )within the regular expression to capture part of the incoming string for use in the outgoing string, where I've reference back to it using $1. An alternative would have been to backreference to it using \1.
(written 2007-06-28, updated 2007-07-06)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P212 - Perl - More on Character Strings
  [453] Commenting Perl regular expressions - (2005-09-30)
  [583] Remember to process blank lines - (2006-01-31)
  [586] Perl Regular Expressions - finding the position and length of the match - (2006-02-02)
  [597] Storing a regular expression in a perl variable - (2006-02-09)
  [608] Don't expose your regular expressions - (2006-02-15)
  [737] Coloured text in a terminal from Perl - (2006-05-29)
  [928] C++ and Perl - why did they do it THAT way? - (2006-11-16)
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [1222] Perl, the substitute operator s - (2007-06-08)
  [1230] Commenting a Perl Regular Expression - (2007-06-12)
  [1305] Regular expressions made easy - building from components - (2007-08-16)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1510] Handling Binary data (.gif file example) in Perl - (2008-01-17)
  [1727] Equality and looks like tests - Perl - (2008-07-29)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1947] Perl substitute - the e modifier - (2008-12-16)
  [2230] Running a piece of code is like drinking a pint of beer - (2009-06-11)
  [2379] Making variables persistant, pretending a database is a variable and other Perl tricks - (2009-08-27)
  [2657] Want to do a big batch edit? Nothing beats Perl! - (2010-03-01)
  [2801] Binary data handling with unpack in Perl - (2010-06-10)
  [2834] Teaching examples in Perl - third and final part - (2010-06-27)
  [2874] Unpacking a Perl string into a list - (2010-07-16)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2993] Arrays v Lists - what is the difference, why use one or the other - (2010-10-10)
  [3059] Object Orientation in an hour and other Perl Lectures - (2010-11-18)
  [3100] Looking ahead and behind in Regular Expressions - double matching - (2010-12-23)
  [3322] How much has Perl (and other languages) changed? - (2011-06-10)
  [3332] DNA to Amino Acid - a sample Perl script - (2011-06-24)
  [3411] Single and double quotes strings in Perl - what is the difference? - (2011-08-30)
  [3546] The difference between dot (a.k.a. full stop, period) and comma in Perl - (2011-12-09)
  [3630] Serialsing and unserialising data for storage and transfer in Perl - (2012-02-28)
  [3650] Possessive Regular Expression Matching - Perl, Objective C and some other languages - (2012-03-12)
  [3707] Converting codons via Amino Acids to Proteins in Perl - (2012-04-25)
  [3927] First match or all matches? Perl Regular Expressions - (2012-11-19)
  [4452] Binary data handling - Python and Perl - (2015-03-09)


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