The document is a comprehensive guide on regular expressions, aimed at developers who may not have prior experience with them. It introduces the topic with essential definitions and gradually builds up to more complex patterns, explaining conventions, metacharacters, character classes, and grouping. The guide emphasizes incremental learning and provides practical examples to demonstrate how to construct regex patterns effectively.

1. Introduction
Diving In
Building up
/regular expressions/demystified
From deckhand to pirate in 30 minutes
Philip Tellis / philip@bluesmoon.info
Yahoo!
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2. Introduction
Diving In
Building up
Outline
1 Introduction
Who’s playing?
Conventions
2 Diving In
Starting Small
Getting meta
3 Building up
More or less
Alternation
Groups
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3. Introduction
Who’s playing?
Diving In
Conventions
Building up
$ whoami?
Philip Tellis
philip@bluesmoon.info
@bluesmoon
yahoo
geek
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4. Introduction
Who’s playing?
Diving In
Conventions
Building up
Who are you?
Developer
Curious
Interested in regular expressions
You may or may not have used them before
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5. Introduction
Who’s playing?
Diving In
Conventions
Building up
What is a regular expression?
A pattern that can match multiple strings
A pattern matching language
A Finite Automaton
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6. Introduction
Who’s playing?
Diving In
Conventions
Building up
What is a regular expression?
But this is a hacker session, so let’s forget the theory.
(You can read the book later.)
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7. Introduction
Who’s playing?
Diving In
Conventions
Building up
What is a regular expression?
But this is a hacker session, so let’s forget the theory.
(You can read the book later.)
/regular expressions/demystified

8. Introduction
Who’s playing?
Diving In
Conventions
Building up
Conventions used in this talk
Text in ’single quotes’ denotes a literal string
Text in /forward slashes/ denotes a regular
expression
The operator =∼ indicates that the string on the left
matches the pattern on the right
The operator !∼ indicates that the string on the left does
not match the pattern on the right
$string denotes a variable containing a string
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9. Introduction
Starting Small
Diving In
Getting meta
Building up
Match a single character
’a’ =~ /a/
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10. Introduction
Starting Small
Diving In
Getting meta
Building up
Let’s try a different character
’t’ =~ /t/
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11. Introduction
Starting Small
Diving In
Getting meta
Building up
Building up
Combine the previous two into a single regular expression
’at’ =~ /at/
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12. Introduction
Starting Small
Diving In
Getting meta
Building up
You now know regular expressions
To build a regular expression, break the pattern into small
manageable pieces and incrementally combine them.
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13. Introduction
Starting Small
Diving In
Getting meta
Building up
Metacharacters
The regex language has its own syntax characters to do
funky things
Some of these act as wild cards
Others act as modifiers to whatever comes before them
And some of them make your brain explode
We won’t be blowing up brains today
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14. Introduction
Starting Small
Diving In
Getting meta
Building up
Metacharacters
The regex language has its own syntax characters to do
funky things
Some of these act as wild cards
Others act as modifiers to whatever comes before them
And some of them make your brain explode
We won’t be blowing up brains today
/regular expressions/demystified

15. Introduction
Starting Small
Diving In
Getting meta
Building up
Metacharacters
The regex language has its own syntax characters to do
funky things
Some of these act as wild cards
Others act as modifiers to whatever comes before them
And some of them make your brain explode
We won’t be blowing up brains today
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16. Introduction
Starting Small
Diving In
Getting meta
Building up
The . metacharacter
Matches ONE and ONLY ONE character
’a’ =~ /./
’b’ =~ /./
’c’ =~ /./
’’ !~ /./
The empty string has less than ONE character
’abc’ has ONE character. . . three times
’abc’ =~ /./
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17. Introduction
Starting Small
Diving In
Getting meta
Building up
The . metacharacter
Matches ONE and ONLY ONE character
’a’ =~ /./
’b’ =~ /./
’c’ =~ /./
’’ !~ /./
The empty string has less than ONE character
’abc’ has ONE character. . . three times
’abc’ =~ /./
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18. Introduction
Starting Small
Diving In
Getting meta
Building up
The . metacharacter
Matches ONE and ONLY ONE character
’a’ =~ /./
’b’ =~ /./
’c’ =~ /./
’’ !~ /./
The empty string has less than ONE character
’abc’ has ONE character. . . three times
’abc’ =~ /./
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19. Introduction
Starting Small
Diving In
Getting meta
Building up
The fate of gate hate date
/.ate/
Matches Does not match
aate bate cate date . . . ate
crates abates dates elated ates ated
...
@ate 9ate ’ ate’
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20. Introduction
Starting Small
Diving In
Getting meta
Building up
The fate of gate hate date
/.ate/
Matches Does not match
aate bate cate date . . . ate
crates abates dates elated ates ated
...
@ate 9ate ’ ate’
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21. Introduction
Starting Small
Diving In
Getting meta
Building up
Character classes
/[a-z]ate/
Matches Does not match
aate bate cate date . . . ate
crates abates dates elated ates ated
... @ate 9ate ’ ate’
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22. Introduction
Starting Small
Diving In
Getting meta
Building up
Character classes
To match a literal ’-’ it should be the first or last character in
the class:
/[+-*/]/ # Incorrect
/[+*/-]/ # Correct
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23. Introduction
Starting Small
Diving In
Getting meta
Building up
Negated character classes
/[^a-z]ate/
Matches Does not match
@ate 9ate ’ ate’ ate ates ated
g@ate e9ated aate bate cate date . . .
crates abates dates elated
...
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24. Introduction
Starting Small
Diving In
Getting meta
Building up
The late fate of gate hate date rate
/[df-hlr]ate/
Matches Does not match
date fate gate hate late ate aate bate cate eate
rate iate jate kate . . .
dates fated billgates hated
...
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25. Introduction
Starting Small
Diving In
Getting meta
Building up
The late fate of gate hate date rate
/[df-hlr]ate/
Matches Does not match
date fate gate hate late ate aate bate cate eate
rate iate jate kate . . .
dates fated billgates hated
...
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26. Introduction
Starting Small
Diving In
Getting meta
Building up
Anchors
/^[df-hlr]ate$/
Matches Does not match
date fate gate hate late ate aate bate . . .
rate dates gated berate elated
...
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27. Introduction
Starting Small
Diving In
Getting meta
Building up
Anchors
ˆ matches the start of the string
$ matches the end of the string
Both are 0 byte matches, ie, they do not match any
character
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28. Introduction More or less
Diving In Alternation
Building up Groups
Matching more than one of something
? – matches 0 or 1 of what comes before it
* – matches 0 or more of what comes before it
+ – matches 1 or more of what comes before it
{n,m} – matches between n and m of what comes before it
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29. Introduction More or less
Diving In Alternation
Building up Groups
Aaargh!
Everyone shout “Aaarrrgh!”
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30. Introduction More or less
Diving In Alternation
Building up Groups
How many ways can you say Aargh!?
argh
aaaaaargh
aaaarrrrghhh
aaaaarrrrrggggghhhh
aaarrrrggggg
aaaaarrrrrhhhh
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31. Introduction More or less
Diving In Alternation
Building up Groups
Match ’em all
/a+r+g+h+/ # aarrrrgggghhhh
/a+r+g+h*/ # aarrgghh & aarrgg
/a+r+g*h+/ # aarrgghh & aarrhh
/a+r+g*h*/ # argh & arg & arh
That last one also matches ’ar’ which we don’t want
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32. Introduction More or less
Diving In Alternation
Building up Groups
Match ’em all
/a+r+g+h+/ # aarrrrgggghhhh
/a+r+g+h*/ # aarrgghh & aarrgg
/a+r+g*h+/ # aarrgghh & aarrhh
/a+r+g*h*/ # argh & arg & arh
That last one also matches ’ar’ which we don’t want
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33. Introduction More or less
Diving In Alternation
Building up Groups
Match ’em all
/a+r+g+h+/ # aarrrrgggghhhh
/a+r+g+h*/ # aarrgghh & aarrgg
/a+r+g*h+/ # aarrgghh & aarrhh
/a+r+g*h*/ # argh & arg & arh
That last one also matches ’ar’ which we don’t want
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34. Introduction More or less
Diving In Alternation
Building up Groups
Match ’em all
/a+r+g+h+/ # aarrrrgggghhhh
/a+r+g+h*/ # aarrgghh & aarrgg
/a+r+g*h+/ # aarrgghh & aarrhh
/a+r+g*h*/ # argh & arg & arh
That last one also matches ’ar’ which we don’t want
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35. Introduction More or less
Diving In Alternation
Building up Groups
Match ’em all
/a+r+g+h+/ # aarrrrgggghhhh
/a+r+g+h*/ # aarrgghh & aarrgg
/a+r+g*h+/ # aarrgghh & aarrhh
/a+r+g*h*/ # argh & arg & arh
That last one also matches ’ar’ which we don’t want
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36. Introduction More or less
Diving In Alternation
Building up Groups
Alternation: Match all this or all that
/ab|cd/
Matches either ’ab’ or ’cd’
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37. Introduction More or less
Diving In Alternation
Building up Groups
From here to eternity
| matches either everything on its left or everything on its right
(That’s a pipe character, not the letter I)
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38. Introduction More or less
Diving In Alternation
Building up Groups
Back to aaargh
/g*h+|g+h*/
This matches all the endings we want:
ggggghhhhhh
ggggg
hhhhh
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39. Introduction More or less
Diving In Alternation
Building up Groups
Back to aaargh
/a+r+g*h+|g+h*/
This doesn’t quite work
Matches Does not match
aaarrrhhh aaarrrggg
aaarrrrggghhh
gggg
gggghhhh
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40. Introduction More or less
Diving In Alternation
Building up Groups
Back to aaargh
/a+r+g*h+|g+h*/
This doesn’t quite work
Matches Does not match
aaarrrhhh aaarrrggg
aaarrrrggghhh
gggg
gggghhhh
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41. Introduction More or less
Diving In Alternation
Building up Groups
Group the subexpression
/a+r+(g*h+|g+h*)/
Matches
aaarrrhhh
aaarrrggg
aaarrrrggghhh
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42. Introduction More or less
Diving In Alternation
Building up Groups
Grouping parentheses
( and ) mark a group
| alternates within a group
Groups may be nested - it’s like a new regex inside
+, *, ? and {n,m} may apply to an entire group
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43. Introduction
Diving In
Building up
Stop
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44. Introduction
Diving In
Building up
Summary
Start small, match the parts you understand
Build up to more complex patterns
Not all problems should be solved by regular expressions
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45. Introduction
Diving In
Building up
More Info. . .
“Mastering Regular Expressions” – Jeffrey Friedl
http://tech.bluesmoon.info/search/label/regex
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46. Introduction
Diving In
Building up
Contact me
Philip Tellis
philip@bluesmoon.info
@bluesmoon
bluesmoon.info
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47. Introduction
Diving In
Building up
Image credits
http://flickr.com/photos/practicalowl/3933514241/
http://flickr.com/photos/loozrboy/3908830690/
http://flickr.com/photos/thetruthabout/2680546103/
http://flickr.com/photos/donsolo/2136923757/
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48. Introduction
Diving In
Building up
Thank You
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49. Introduction
Diving In
Building up
Aargh with class
/a+r+g*[gh]h*/
Matches
aaarrrhhh
aaarrrggg
aaarrrrggghhh
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50. Introduction
Diving In
Building up
Matching meta characters in a character class
/[a-zA-Z0-9_-]/
/[a-z^]/
/[][]/
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51. Introduction
Diving In
Building up
Alternating multiple items
/apples|oranges|bananas/
/buy some (apples|oranges|ba(na){2}s)/
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