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Regular expressions

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A regular expression (or regex) is a string of characters, (some of which being reserved control characters,) which represent a pattern [1], i.e. a string designed to match a particular sequence of characters. Regular expressions provide the basic tool in searching, and are ubiquitous in the electronic world.

Getting started

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There are many editors with regex functionalities. Here are a few examples (Please feel free to add or remove if you find better ones.)

Learning materials

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A lightning introduction

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There are several "dialects" (e.g. javascript, perl, php, python) of regular exprssions which differ slightly in grammar. Let us focus on python regex for the moment (because I happen to have a reference [2] for it).

Control characters

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  • Python regex has the control characters :

\-.*+?$<!=|()[]^:#

First examples

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[please verify]

  • Any string (e.g. abcdefg)which does not contain any control characters is trivially a regular expression ("regex") pattern. It matches only itself
  • The pattern [A-Z] matches a character between A and Z (in the ASCII table)
  • A backslash (\) followed by any control character, such as \. or even the backslash itself \\, match the character itself (this pattern is called an "escape"). In our examples, \. matches the single dot . and \\ matches the backslash
  • Combining the two examples above, the pattern [A-Za-z0-9\-] matches any single alphanumeric character or the dash "-".
  • The pattern \n matches a newline
  • The pattern abc.xyz matches a string that starts with abc, then contains any character except an end-of-line character, then ends with xyz
  • The pattern a* matches a string with as many characters "a" as possible; it also matches the empty string "".
  • Combining the previous two examples, we get a very common pattern: abc.*xyz matches a string which starts and ends with "abc" and "xyz" respectively, and between which is the longest available string (which could be empty) of any character except the newline.

Exercises

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  • Question: What is [A-Za-z0-9\-]?
  • Write a regular expression to match (a) the URL of any wikiversity page; (b) the URL for any page on any wikimedia site, and (c) the email address of all your friends. Check with a regex editor that your regex actually works.

Write your proposed solutions below

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Further lessons

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[proposals]

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Notes

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  1. ↑ Martelli, Python in a nutshell, p.203
  2. ↑ Alex Martelli, Python in a nutshell ISBN 0596100469
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