Regular expressions are under-valued and most developers tend to only know the basics. Having a thorough understanding of how regular expressions work, will be incredibly helpful when you need to parse structured data. This presentation will assume you already know what regular expressions are, but will sum up (with an example) some fancy things you probably didn’t know were possible with regular expressions. If you're interested in a more detailed write-up, I suggest you check out http://www.mullie.eu/regular-expressions-basics/ & http://www.mullie.eu/regular-expressions-advanced/ This presentation is based on the PHP-implementation of PCRE, but nearly all programming languages support the same functionality, albeit sometimes with their own twists.

1. REGEX 101
The Swiss Army knife of string manipulation

2. @matthiasmullie
Regular expressions 101

3. INTRODUCTION
What are regular expressions?
Regular expressions 101

4. Regular expressions are special
characters that match or capture
portions of a field, as well as the rules
that govern all characters.
Google
Regular expressions 101 » Introduction

5. A regular expression provides a
concise and flexible means for
"matching" strings of text, such as
particular characters, words, or
patterns of characters.
Wikipedia
Regular expressions 101 » Introduction

6. /{$([a-z0-9_]*)((.[a-z0-9_]*)*)
(->[a-z0-9_]*((.[a-z0-9_]*)*))?
((|[a-z_][a-z0-9_]*(:.*?)*)*)}/i
Regular expressions 101 » Introduction

7. Regular expressions find patterns in
strings.
Me
Regular expressions 101 » Introduction

8. Neque porro quisquam est qui
dolorem ipsum quia dolor sit amet,
consectetur, adipisci velit...
!
!
‣
/ipsum/
‣
/[a-z]/i
‣
/(est|qui)/
‣
/[^w]/i
Regular expressions 101 » Introduction

9. BASICS
The syntax everyone should know already
Regular expressions 101

10. /Delimiter/
‣
‣
Any [^a-zA-Z0-9s] character
Opening char == terminating char
‣ Except for [ ], ( ), { } and < >
Regular expressions 101 » Delimiter

11. Use /
(uniformity, you know)
Regular expressions 101 » Delimiter

12. Meta characters
‣
.
‣
( )
‣
[ ]
‣
‣
^
‣
*
‣
|
‣
{n}
$
?
Regular expressions 101 » Meta characters
+
{n,m}

13. Pattern modifiers //x
‣
i
‣
A
‣
m
‣
D
‣
s
‣
U
‣
x
‣
J
‣
e
‣
...
Regular expressions 101 » Pattern modifiers

14. Character classes [ ]
Ranges
Inverse ranges
‣
[0-9]
‣
[^0-9]
‣
[a-zA-Z]
‣
[^a-zA-Z]
‣
[A-F0-9]
‣
[^A-F0-9]
Regular expressions 101 » Character classes

15. Character classes [ ]
No sequence of characters!
!
‣
l, o, r, e or m
‣
[lorem]
lorem
Regular expressions 101 » Character classes

16. Character classes [ ]
POSIX
‣
[:alnum:]
‣
[:blank:]
‣
[:lower:]
‣
...
Regular expressions 101 » Character classes

17. Greediness: greedy
<ul><li>list-item1</li><li>list-item2</li></ul>
!
/<li>.*</li>/
‣
<li>list-item1</li><li>list-item2</li>
Regular expressions 101 » Greediness

18. Greediness: lazy
<ul><li>list-item1</li><li>list-item2</li></ul>
!
/<li>.*?</li>/
or
‣
<li>list-item1</li>
‣
/<li>.*</li>/U
<li>list-item2</li>
Regular expressions 101 » Greediness

19. Subpatterns
/([a-z0-9]*)@([a-z0-9.]*.[a-z0-9]{2,3})/i
!
user
email
hostname
Note: this regex only barely satisfies my needs for this particular example; do not use this really find occurrences of email addresses, it does not fully satisfy RFC5321 & RFC5322
Regular expressions 101 » Subpatterns

20. Questions?
Regular expressions 101

21. ADVANCED
The juicy stuff you never knew about, until now
Regular expressions 101

22. Back references
Problem: /href=['"](.*?)['"]/i
!
Matches:
!
‣
href="xxx"
‣
href="xxx'
‣
href='xxx'
‣
href='xxx"
Regular expressions 101 » Back references

23. Back references
Solution: /href=(['"])(.*?)1/i
1 references first subpattern!
!
Don’t forget to also string-escape in PHP:
preg_match('/href=(['"])(.*?)1/i', ...);
Regular expressions 101 » Back references

24. Named subpatterns
Scenario: parsing large CSV
1,a title,5.00,92,green
2,another title,3.50,4,blue
3,one more,33699.99,15,white
...
Regular expressions 101 » Named subpatterns

25. Named subpatterns
/([0-9]+),(.*?),([0-9]+.[0-9]{2}),([0-9]+),([a-z]+)/i
!
!
Result excerpt:
[1]
[2]
[3]
[4]
[5]
=>
=>
=>
=>
=>
string(1)
string(7)
string(4)
string(2)
string(5)
!
!
!
!
Regular expressions 101 » Named subpatterns
"1"
"a title"
"5.00"
"92"
"green"

26. Named subpatterns
/(?P<id>[0-9]+),(?P<title>.*?),(?P<price>[0-9]+.[0-9]
{2}),(?P<stock>[0-9]+),(?P<color>[a-z]+)/i
!
Result excerpt:
["id"] => string(1) "1"
[1] => string(1) "1"
["title"] => string(7) "a title"
[2] => string(7) "a title"
["price"] => string(4) "5.00"
[3] => string(4) "5.00"
["stock"] => string(2) "92"
[4] => string(2) "92"
["color"] => string(5) "green"
[5] => string(5) "green"
Regular expressions 101 » Named subpatterns

27. Named subpatterns
‣
(?P<name>pattern)
‣
(?<name>pattern) & (?'name'pattern)
since PHP 5.2.2
Regular expressions 101 » Named subpatterns

28. Named subpatterns +
back references
!
/href=(?P<quotes>['"])(?P<href>.*?)(?P=quotes)/i
Regular expressions 101 » Named subpatterns

29. Lookahead/-behind assertions
“Take a peek, don’t eat it”
Regular expressions 101 » Assertions

30. Lookahead/-behind assertions
Scenario: find all occurrences of “here”
!
“Where can I find here, not there?”
Regular expressions 101 » Assertions

31. Lookahead/-behind assertions
Deduction:
Find all here’s, not preceded or followed by
an alphabetic character.
!
Solution: /(?<![a-z])here(?![a-z])/i
Regular expressions 101 » Assertions

32. Lookahead/-behind assertions
‣
Positive lookahead: (?=expression)
‣
Negative lookahead: (?!expression)
‣
Positive lookbehind: (?<=expression)
‣
Negative lookbehind: (?<!expression)
Regular expressions 101 » Assertions

33. Lookahead/-behind assertions
“lookbehind assertion is not fixed length...”
In PHP, lookbehind can not contain repetition,
while lookahead can.
‣
(?=.*)
‣
(?<=.*)
‣
(?=abc)
‣
(?<=abc)
Regular expressions 101 » Assertions

34. Conditional subpatterns
if-then(-else) in regular expressions
!
!
YES RLY!
Regular expressions 101 » Conditional subpatterns

35. Conditional subpatterns
Scenario: match all (x|ht)ml tags
!
Caution!
‣ <element></element>
‣
<element />
Regular expressions 101 » Conditional subpatterns

36. Conditional subpatterns
Solution:
if then else
/<(?P<tag>[a-z]+).*?(?P<self>/)?>(?(self)|.*?</(?P=tag)>)/i
Named patterns
If self-closing, then do nothing,
else, find matching end tag
Regular expressions 101 » Conditional subpatterns

37. Conditional subpatterns
With subpattern (named or by id):
‣
‣
(?(pattern)then)
‣
(?(pattern)then|else)
With lookahead/-behind:
‣
‣
(?(?=assertion)then)
‣
(?(?=assertion)then|else)
Regular expressions 101 » Conditional subpatterns

38. Comments
/
# match currency symbols for USD, EUR, GBP & YEN
[$€£¥]
# must be followed by a number to indicate a price
(?=[0-9])
# pattern modifiers:
# u for UTF-8 interpretation (currency symbols),
# x to ignore whitespace (for comments)
/ux
Regular expressions 101 » Comments

39. Comments
‣
# Perl-style
‣
/x modifier
‣
Ignores unescaped whitespace
Regular expressions 101 » Comments

40. Presentation title

41. Questions?
Regular expressions 101

42. Resources
‣
www.mullie.eu/regular-expressions-basics/
‣
www.mullie.eu/regular-expressions-advanced/
mullie.eu
Regular expressions 101