Regular Expressions

Internal presentation on regular expressions, 2012

Transcript

1. © John Chandler 2012 Regular Expressions
2. http://xkcd.com/208/
3. What? A pattern describing a set of text strings
4. When? From formal regular set theory in 1950s Popularised by beardy UNIX types in 1970s
5. Why? Matching / searching text Search and replace Parsing strings
6. Main Flavours POSIX Basic / Extended (default with UNIX tools) Perl-derived – very popular! (Perl, Python, PHP, JavaScript, .NET)
7. Syntax: The First 90% c standard characters . any character ^ start of string $ end of string c* zero or more of c
8. Syntax: The First 90% ^Pip*e.*$ matches: Piper at the Gates of Dawn Pie and Chips
9. Syntax: The First 90% ^Pip*e.*$ doesn't match: A Pipe pie spam chips and spam
10. Choices | or, alternative [abc] character class [^abc] inverse of class
11. Choices foo|bar matches: foo fighters toolbar bar fool
12. Special Sequences \b beginning / end of word \d decimal digit \s whitespace \w alphanumeric and underscore
13. Repetition s * zero or more + one or more ? zero or one {n} exactly n-times {n,m} n to m times
14. Groups (abc) capturing group (?:abc) non-capturing group (?P<name>) named capturing group (?P=name) match a previous match
15. What do these do? (foo|bar) ((?:abc){3,4}) (?P<foo>\w+) [“']([^”']+)[“']
16. What about these? (fish) \1 (?P<thing>fish) (?P=thing) Isaac (?=Asimov) Isaac (?!Asimov)
17. Greedy Matching By default, regular expressions are greedy. They will find the biggest match that satisfies an expression.
18. Non-greedy Repetitions *? zero or more +? one or more ?? zero or one
19. Writing Regexes Don't if there is a simpler way Build regex from small chunks Use the Python and PHP REPLs Document!
20. Debugging Regexes Don't panic Break regex into smaller chunks Use the Python and PHP REPLs Watch out for special characters!
21. Anything else? Any questions? Read the Python and PHP docs Try the UNIX grep command