Be gentle rather than macho ... regular expression techniques

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 Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own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

Please don't be "macho" with your regular expressions.

You can write a very long and complex pattern to match something like a postcode or email address, but if you do, it's be hard to test, difficult to debug, and awkard to maintain. And to add insult to injury, it will probably run slower that the alternative approach.

"What alternative approach?" I hear you asking.

Softly, softly.

Just as you would (manuall) look at an email address quickly and say "does it have an @ symbol in the middle", do the same thing with an easier regular expression that a full, detailed match. Capture the bit before the @ (the user name) into one variable, and the bit after the @ (the domain name) into another. And only then check each of them to see if they match (a much simpler) pattern.

From last week's Python course - an example [full source]:

eare = re.compile(r"^\s*(\S+)@(\S+)\s*$")

emmas_bits = eare.findall(emma)

domain_els = emmas_bits[0][1].split(".")

The first line breaks the email address down into two chunks which it stores in the emmas_bits collection. Then the split separates the elements of the domain name at the "." characters. Much easier to read and write than trying to do it all in one!
(written 2010-08-08)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles

Q805 - Object Orientation and General technical topics - Advanced Regular Expression Components
  [728] Looking ahead and behind in a Regular Expression - (2006-05-22)
  [3089] Python regular expressions - repeating, splitting, lookahead and lookbehind - (2010-12-17)
  [3100] Looking ahead and behind in Regular Expressions - double matching - (2010-12-23)
  [3790] Solution looking for a problem? Lookahead and Lookbehind - (2012-06-30)

Q803 - Object Orientation and General technical topics - Regular Expressions - Extra Elements
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1372] A taster PHP expression ... - (2007-09-30)
  [1601] Replacing the last comma with an and - (2008-04-04)
  [1613] Regular expression for 6 digits OR 25 digits - (2008-04-16)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1860] Seven new intermediate Perl examples - (2008-10-30)
  [3516] Regular Expression modifiers in PHP - summary table - (2011-11-12)
  [3650] Possessive Regular Expression Matching - Perl, Objective C and some other languages - (2012-03-12)

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Some other Articles

Six languages in one file - an HTML++ web page
Predictions for the seagull population
Feeding the Grockles
Robust - testing the system
Be gentle rather than macho ... regular expression techniques
How Green is my train?
Dartmouth, Kingswear and Torbay
Switching from Adobe Photoshop to Gimp - testing it on animals
Defining static methods in Python
Melksham Area Board meeting of last week

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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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