Regular Expressions 101
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Regular expressions (RegEx) are patterns used to match character combinations in strings. They were invented in the 1950s and exist in different flavors that may implement patterns differently. RegEx can be used to search, edit, and manipulate strings in programming languages and text editors. Common patterns include characters, character classes, quantifiers, anchors, groupings, lookarounds, and modifiers. Mastering RegEx provides powerful string searching and manipulation capabilities.
![Meta Characters
Special characters : )(][^$.|?*+
To match as literals, escape them
with a backslash](https://image.slidesharecdn.com/regularexpressions101-110721045036-phpapp01/85/Regular-Expressions-101-25-320.jpg)
![Character Classes
Matches one and ONLY one character in
a set of characters
[Aa] : matches either ‘A’ or ‘a’
[a-z] : matches any of the lowercase
alphabets in the specified range ONCE
[^Aa] : matches anything but ‘A’ and
‘a’](https://image.slidesharecdn.com/regularexpressions101-110721045036-phpapp01/85/Regular-Expressions-101-26-320.jpg)
![Character Classes
Metacharacters may behave differently
within character classes
[^red] : matches anything but ‘r’,
‘e’ and ‘d’
[r^ed] : matches only ‘r’, ‘^’, ‘e’
or ‘d’](https://image.slidesharecdn.com/regularexpressions101-110721045036-phpapp01/85/Regular-Expressions-101-27-320.jpg)
![Shorthand Classes
d, [0-9]: digits
w, [da-zA-Z_]: alphanumeric or _
s or [ t(?:n|rn)] : whitespace
D, W, S : the above BUT negated](https://image.slidesharecdn.com/regularexpressions101-110721045036-phpapp01/85/Regular-Expressions-101-28-320.jpg)
![The Dot Character
The Dot (.) character matches any
single character BUT the newline
Synonymous with [^n] (UNIX/Linux/
Mac)
as well as [^rn] (Windows)
Use it sparingly - it’s expensive](https://image.slidesharecdn.com/regularexpressions101-110721045036-phpapp01/85/Regular-Expressions-101-29-320.jpg)
Regular Expressions 101
- 1. Regular Expressions 101 Raj Kissu
- 2. What are Regular Expressions?
- 3. They describe patterns in strings
- 4. These patterns can be used to modify strings
- 5. Invented by Stephen Cole Kleene
- 6. Idea of RegEx dates back to the 1950s
- 7. Today, they come in different “flavors”
- 8. PCRE, POSIX Basic & Extended RegEx, ECMA RegEx and loads more!
- 9. NOTE: RegEx “flavors” are not consistent in implementation
- 10. Different “flavors” behave differently ...
- 11. So know your “flavor” before you use it!
- 12. Why Use It?
- 13. Cos it’s an important tool in every coder’s arsenal!
- 14. Ok, but still ... Why use it???
- 15. It’s Short, Sweet & F***ing Fast!
- 16. It can match just about anything!
- 17. It makes changing large amounts of repetitive text trivial ...
- 18. ... as long as you can “see” patterns
- 19. It Makes You Awesome - in a Geeky way :)
- 22. Readily Available Support in programming languages: JavaScript, PHP, PERL, C/C++,etc Command-line: grep, awk, sed Text-editors: VIM, emacs, Notepad++ IDEs: Aptana, Netbeans, Visual Studio .NET
- 23. RegEx Basics NOTE : Using ECMA (JavaScript) RegEx Flavor!
- 24. Characters standard characters letters : A to Z, a to z numbers : 0 to 9 symbols : !,@,#,%,& etc Matched literally!
- 25. Meta Characters Special characters : )(][^$.|?*+ To match as literals, escape them with a backslash
- 26. Character Classes Matches one and ONLY one character in a set of characters [Aa] : matches either ‘A’ or ‘a’ [a-z] : matches any of the lowercase alphabets in the specified range ONCE [^Aa] : matches anything but ‘A’ and ‘a’
- 27. Character Classes Metacharacters may behave differently within character classes [^red] : matches anything but ‘r’, ‘e’ and ‘d’ [r^ed] : matches only ‘r’, ‘^’, ‘e’ or ‘d’
- 28. Shorthand Classes d, [0-9]: digits w, [da-zA-Z_]: alphanumeric or _ s or [ t(?:n|rn)] : whitespace D, W, S : the above BUT negated
- 29. The Dot Character The Dot (.) character matches any single character BUT the newline Synonymous with [^n] (UNIX/Linux/ Mac) as well as [^rn] (Windows) Use it sparingly - it’s expensive
- 30. Alternation Using a pipe |, match either the left or right side of the pattern bear|tiger : matches a string that contains either “bear” or “tiger” pedo(bea|tige)r : matches a string that contains either “pedobear” or “pedotiger”
- 31. Quantifiers {n} : matches exactly n times {n,} : matches n or more times {n,m} : matches between n and m times * : same as {0,} + : same as {1,} ? : same as {0,1}
- 32. Quantifiers Quantifiers are greedy <.+> : matches “<div>holy RegEx, Batman!</div>” instead of stopping at “<div>” Add a ? to make it lazy <.+?> : stops at “<div>” in “<div>holy regex!</div>”
- 33. Anchors Matches positions instead of characters ^ : matches the beginning of a string $ : matches the end of a string b : matches between a w and a token that’s not a w
- 34. Groupings Placing parentheses around tokens groups them together : /nyan(cat)/ It also provides a backreference : /(cat)1/ matches “cat” OR if you don’t want a backreference : /(?:nyan)(cat)1/ matches “nyancatcat” and not “nyancatnyan”
- 35. Lookahead Positive Lookahead Iron(?=man) : matches “Iron” only if it is followed by “man” Negative Lookahead Iron(?!man) : matches “Iron” only if it is not followed by “man”
- 36. Lookbehind Positive Lookbehind (?<=Iron)man : matches “man” only if it is preceded by “Iron” Negative Lookbehind (?<!Iron)man : matches “man” only if it is not preceded by “Iron”
- 37. Modifiers alter behavior of the matching mode (differs between tools) /i : case-insensitive match /m : Multi-line mode /g : affects all possible matches, not just the first
- 38. Q & A
- 39. Resources Mastering Regular Expressions - Jeffrey E.F. Friedl http://www.regex.info http://www.regular-expressions.info http://www.rubular.com
- 40. Thank You!