Regular expressions made easy - building from components

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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 20:25:39)

Associated topics are indexed under

Y115 - Additional Python Facilities
Q806 - Regular Expression Cookbook
R109 - Ruby - Strings and Regular Expressions
T247 - Tcl/Tk - Advanced Regular Expressions
P212 - Perl - More on Character Strings

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