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.

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 :)

20. Before RegEx Mastery

21. After RegEx Mastery

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!