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Regular expressions made easy - building from components

There seems to be a certain macho desire in many programmer's minds to write a single complicated regular expression to match against an input line, ignorning the structured approach that everyone accepts quite cheerfully in almost every other case. Have a look at this Python line:

wholeline = r"\d\d-...-\d\d\d\d\s+(\d\d):(\d\d):(\d\d.\d\d),\s+(-?\d+\.\d+),\s+(-?\d+\.\d+),(-?\d+\.\d+),\s+(-?\d+\.\d+),(-?\d+\.\d+),\s+(-?\d+\.\d+)"

Impressive, isn't it?
Yes
Easy to follow, isn't it?
No!

Much better to build it up from a series of components:

date = r"\d\d-...-\d\d\d\d"
time = r"(\d\d):(\d\d):(\d\d.\d\d)"
whitespace = r"\s+"
floater = r"(-?\d+\.\d+)"
wholeline = date + whitespace + time + "," + whitespace + \
  floater + "," + whitespace + floater + "," + floater + \
  "," + whitespace + floater + "," + floater + "," + \
  whitespace + floater


These examples are from the Python Course I have just concluded - the full example is here - where a log file was to be analysed and a short report generated to highlight any changes in readings of over 1% from one line of the data to the next in any of the data columns.

(written 2007-08-16)

 
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  [3927] First match or all matches? Perl Regular Expressions - (2012-11-19)
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  [3332] DNA to Amino Acid - a sample Perl script - (2011-06-24)
  [3322] How much has Perl (and other languages) changed? - (2011-06-10)
  [3100] Looking ahead and behind in Regular Expressions - double matching - (2010-12-23)
  [3059] Object Orientation in an hour and other Perl Lectures - (2010-11-18)
  [2993] Arrays v Lists - what is the difference, why use one or the other - (2010-10-10)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [2874] Unpacking a Perl string into a list - (2010-07-16)
  [2834] Teaching examples in Perl - third and final part - (2010-06-27)
  [2801] Binary data handling with unpack in Perl - (2010-06-10)
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  [2379] Making variables persistant, pretending a database is a variable and other Perl tricks - (2009-08-27)
  [2230] Running a piece of code is like drinking a pint of beer - (2009-06-11)
  [1947] Perl substitute - the e modifier - (2008-12-16)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1727] Equality and looks like tests - Perl - (2008-07-29)
  [1510] Handling Binary data (.gif file example) in Perl - (2008-01-17)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1251] Substitute operator / modifiers in Perl - (2007-06-28)
  [1230] Commenting a Perl Regular Expression - (2007-06-12)
  [1222] Perl, the substitute operator s - (2007-06-08)
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [928] C++ and Perl - why did they do it THAT way? - (2006-11-16)
  [737] Coloured text in a terminal from Perl - (2006-05-29)
  [608] Don't expose your regular expressions - (2006-02-15)
  [597] Storing a regular expression in a perl variable - (2006-02-09)
  [586] Perl Regular Expressions - finding the position and length of the match - (2006-02-02)
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Q806 - Regular Expression Cookbook
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  [3218] Matching a license plate or product code - Regular Expressions - (2011-03-28)
  [2804] Regular Expression Myths - (2010-06-13)
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  [1613] Regular expression for 6 digits OR 25 digits - (2008-04-16)
  [1412] Sparse and Greedy matching - Tcl 8.4 - (2007-10-27)
  [1410] Tcl / regsub - changing a string and using interesting bits - (2007-10-27)

Y115 - Additional Python Facilities
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  [4593] Command line parameter handling in Python via the argparse module - (2015-12-08)
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  [4085] JSON from Python - first principles, easy example - (2013-05-13)
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