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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 Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own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))
Storing a regular expression in a perl variable

When you match to a regular expression in a perl program, the program has to compile the regular expression (i.e. work out what it does) before actually doing the matching. It's pretty smart about that - it stored the result of than compile so that it only has to do it once during each run. However, if the regular expression contains a variable, Perl is slowed right down as the variable could change and Perl has to recompile every time.

If the variable isn't, in fact, going to change very often during a run you can control the compiling yourself by using a scalar variable to store the compiled regular expression using qr, as shown in this example:

  
$local = "TR|PL|EX|TQ|TA|SN|BA|BS|DT|BH|GL";
$pcode = qr/^\s*($local)\d\w?\s+\d[A-Z]{2}\s*$/;

@vcheck = ("SN12 6QL","G3 7XR","GLZ 7PX"," OX11 0EY","NW1 1AD");

foreach $tp (@vcheck) {
$tp =~ $pcode and print "$1\n";
}


This gave me the single result "SN" as being the only valid postcode within the areas listed in $local.
(written 2006-02-09, updated 2006-06-05)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
P669 - Perl - Data Munging
  [1316] Filtering and altering Perl lists with grep and map - (2007-08-23)
  [1509] Extracting information from a file of records - (2008-01-16)
  [1947] Perl substitute - the e modifier - (2008-12-16)
  [2129] Nothing beats Perl to solve a data manipulation requirement quickly - (2009-04-14)
  [2702] First and last match with Regular Expressions - (2010-04-02)
  [3335] Practical Extraction and Reporting - (2011-06-26)
  [3707] Converting codons via Amino Acids to Proteins in Perl - (2012-04-25)
  [3764] Shell, Awk, Perl of Python? - (2012-06-14)
  [4620] Perl 6 - a Practical Extraction and Reporting example! - (2016-01-11)

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)
  [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)
  [1251] Substitute operator / modifiers in Perl - (2007-06-28)
  [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)
  [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)
  [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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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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