WebFund 2014W Lecture 20: Difference between revisions
Created page with "Basics of Regular Expressions * start and end: / * . represents any single character * * is 0 or more repeats, + is one or more repeats * Thus .* matches any number of charact..." |
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Chomsky hierarchy | Chomsky hierarchy | ||
Regular expressions are used for '''input sanitization''' | |||
* make sure externally provided input is "safe" | |||
* two strategies: whitelists and blacklists | |||
** whilelist: explicit list of "good" things | |||
** blacklist: explicit list of "bad" things | |||
* use whitelists when possible | |||
* whitelists are more work because if you leave anything out things break | |||
Revision as of 18:19, 26 March 2014
Basics of Regular Expressions
- start and end: /
- . represents any single character
- * is 0 or more repeats, + is one or more repeats
- Thus .* matches any number of characters (including none)
- () denote groups, normally for extraction or later substitution
- Each group is numbered, so first () is $1 (or something like that)
- can include letter ranges in [], e.g. [a-z]
- An all lowercase word with at least one character is: /[a-z]+/
- | means or (as usual), and is implicit
Apparently there are regular expression decoders online somewhere
Escaped characters
- \ is used to treat special characters as literals
- % followed by hex numbers denotes character codes
Chomsky hierarchy
Regular expressions are used for input sanitization
- make sure externally provided input is "safe"
- two strategies: whitelists and blacklists
- whilelist: explicit list of "good" things
- blacklist: explicit list of "bad" things
- use whitelists when possible
- whitelists are more work because if you leave anything out things break