Presentation I gave to the local http://www.cocoacoder.org/ meeting on using Regular Expression in Cocoa code (although much of it applies to other languages as well).

1. Regular Expressions
How not to turn one problem into two.
Carl Brown
CarlB@PDAgent.com

2. “Common Wisdom”
“Some people, when confronted
with a problem, think ‘I know, I'll
use regular expressions.’ Now
they have two problems.”
*See http://regex.info/blog/2006-09-15/247 for source.

3. What is a ‘Regular
Expression’?
“...a concise and flexible means for
‘matching’ (specifying and recognizing) strings
of text, such as particular characters, words,
or patterns of characters” (So says Wikipedia)
“... a way of extracting substrings from text in a
‘usefully fuzzy’ way” (So says me)

4. ...so for example?
Pull out the host from a URL string:
http://([^/]*)/
find the date in a string
([0-9][0-9]*[-/][0-9][0-9]*[-/][0-9][0-9]*)

5. But they’re a Pain to
Use
Aren’t they?

6. Two Kinds of (OOish)
Languages
Some languages, Like perl or ruby, have
Regex build into their strings, so they get used
often.
Most others, like Cocoa, Java, Python have
Regular Expression Objects, that are
complicated and a Pain in the Ass

7. Ruby
string.sub(“pattern”,“replacement”)

8. Cocoa (Apple)
+[NSRegularExpression regularExpressionWithPattern:(NSString *)
pattern options:(NSRegularExpressionOptions)options error:(NSError
**) error]
-[NSRegularExpression replaceMatchesInString:(NSMutableString *)
string options:(NSMatchingOptions)options range:(NSRange)range
withTemplate:(NSString *)template]

9. Cocoa (Apple)
+[NSRegularExpression regularExpressionWithPattern:(NSString *)
pattern options:(NSRegularExpressionOptions)options error:(NSError
**) error]
-[NSRegularExpression replaceMatchesInString:(NSMutableString *)
string options:(NSMatchingOptions)options range:(NSRange)range
withTemplate:(NSString *)template]
NSRegularExpressionOptions?
NSMatchingOptions?
Why do I need a Range?
What’s a template string?

10. Cocoa (Apple)
+[NSRegularExpression regularExpressionWithPattern:(NSString *)
pattern options:(NSRegularExpressionOptions)options error:(NSError
**) error]
-[NSRegularExpression replaceMatchesInString:(NSMutableString *)
string options:(NSMatchingOptions)options range:(NSRange)range
withTemplate:(NSString *)template]
NSRegularExpressionOptions?
NSMatchingOptions?
Why do I need a Range?
What’s a template string?
Is it really worth it?

11. Cocoa (sane)
#import "NSString+PDRegex.h"
[string stringByReplacingRegexPattern:@"pattern"
withString:@"replacement" caseInsensitive:NO];
*See https://github.com/carlbrown/RegexOnNSString/

12. Python
(an aside)
import re
re.match(“pattern”,“a pattern”) #no match
re.search(“pattern”,“a pattern”) #matches fine

13. But Regex’s are
impossible to maintain...
Aren’t they?

14. But what about?
(?<!(=)|(="")|(='))(((http|ftp|
https)://)|(www.))+[w]+(.[w]+)
([w-.@?^=%&amp;:/~+#]*[w-@?
^=%&amp;/~+#])?(?!.*/a>)

15. But what about?
(?<!(=)|(="")|(='))(((http|ftp|
https)://)|(www.))+[w]+(.[w]+)
([w-.@?^=%&amp;:/~+#]*[w-@?
^=%&amp;/~+#])?(?!.*/a>)
*That* guy has two problems

16. But what about?
(?<!(=)|(="")|(='))(((http|ftp|
https)://)|(www.))+[w]+(.[w]+)
([w-.@?^=%&amp;:/~+#]*[w-@?
^=%&amp;/~+#])?(?!.*/a>)
*That* guy has two problems
Well, Actually, he has n! problems where,
n is the number of hyperlinks in the input string

17. How to keep that from
happening (my advice)
Limit yourself to only the basic meta-
characters.
Favor clarity over brevity.
Take more smaller bites.
Beware of greedy matching

18. The Basic Characters
A Phrasebook

19. PhraseBook pt 1

20. PhraseBook pt 1
^.*
“the junk to the left of what I want”
This breaks down as ^ (the beginning of the string)
followed by .* any number of any character.

21. PhraseBook pt 1
^.*
“the junk to the left of what I want”
This breaks down as ^ (the beginning of the string)
followed by .* any number of any character.
.*$
“the junk to the right of what I want”
This breaks down as any number of any character .*
followed by $ (the end of the string)

22. PhraseBook pt 2
[0–9][0–9]*
“a number with at least one digit”
The brackets ([ and ]) mean “any of the characters contained
within the brackets”. So this means 1 character of 0–9 (so 0 1 2
3 4 5 6 7 8 or 9) followed by zero or more of the same character.

23. PhraseBook pt 2
[0–9][0–9]*
“a number with at least one digit”
The brackets ([ and ]) mean “any of the characters contained
within the brackets”. So this means 1 character of 0–9 (so 0 1 2
3 4 5 6 7 8 or 9) followed by zero or more of the same character.
[^A-Za-z]
“any character that’s not a letter”
The ^ as the first character inside the brackets means “not” so
instead of meaning “any letter” it means “anything not a letter”.

24. PhraseBook pt 3
.
“a literal period” (e.g. to match the dot in .com)

25. PhraseBook pt 3
.
“a literal period” (e.g. to match the dot in .com)
*
“a literal * ” (e.g. to match an asterisk)

26. PhraseBook pt 3
.
“a literal period” (e.g. to match the dot in .com)
*
“a literal * ” (e.g. to match an asterisk)
( ) or [ ]
“literal parenthesis/brackets” (in Cocoa, at least)

27. PhraseBook pt 3
.
“a literal period” (e.g. to match the dot in .com)
*
“a literal * ” (e.g. to match an asterisk)
( ) or [ ]
“literal parenthesis/brackets” (in Cocoa, at least)
( …stuff… )
“stuff I want to refer to later as $1” (in Cocoa, at least)

28. PhraseBook pt 4

29. PhraseBook pt 4
There is no...
Part 4

30. But what about?
* Cheat Sheet from http://www.addedbytes.com/cheat-sheets/regular-expressions-cheat-sheet/

31. But what about?

32. There is no...
Part 4
But what about?

33. Clarity > Brevity
(Really true of any language)

34. Choose the clearest
way:
[A-Za-z_] instead of w
[^A-Za-z_] instead of W

35. Choose the
consistent way:

36. Choose the
consistent way:
OSX:~$ grep '^root::*' /etc/passwd
root:*:0:0:System Administrator:/var/root:/bin/sh
OSX:~$ grep '^root:+' /etc/passwd
OSX:~$

37. Choose the
consistent way:
OSX:~$ grep '^root::*' /etc/passwd
root:*:0:0:System Administrator:/var/root:/bin/sh
OSX:~$ grep '^root:+' /etc/passwd
OSX:~$
OSX:~$ grep '^root:.*' /etc/passwd
root:*:0:0:System Administrator:/var/root:/bin/sh
OSX:~$ grep '^root:.*?' /etc/passwd
OSX:~$

38. Except when you
can’t
([^/][^]*)/ => 1
http:// (POSIX/sed)
([^/][^]*)/ => $1
http:// (perl/cocoa)

39. Take Smaller Bites
The less you do at a time, the safer each step is

40. Which is clearer?
NSString *domainName = [myHTMLString
stringByReplacingRegexPattern:
@"^.*href=[”’]http://(.*)/.*$"
withString:@"$1" caseInsensitive:YES];

41. Which is clearer?
NSString *leftRemoved = [myHTMLString
stringByReplacingRegexPattern: @"^.*href=[‘“]"
withString:@"" caseInsensitive:YES];
NSString *myURL = [leftRemoved
stringByReplacingRegexPattern: @"[“‘].*$" withString:@""
caseInsensitive:NO];
NSString *hostAndPath = [myURL
stringByReplacingRegexPattern: @"^.*http://"
withString:@"" caseInsensitive:YES];
NSString *domainName = [hostAndPath
stringByReplacingRegexPattern: @"/.*$" withString:@""
caseInsensitive:NO];
Bonus: This one can be stepped through with the debugger :-)

42. But isn’t that slower?
Yes.

43. But isn’t that slower?
Yes.
But it doesn’t matter how fast you get the
wrong answer.

44. Beware Greedy
Matching
Remember this?
NSString *domainName = [myHTMLString
stringByReplacingRegexPattern:
@"^.*href=[”’]http://(.*)/.*$" withString:@"$1"
caseInsensitive:YES];

45. Beware Greedy
Matching
Remember this?
NSString *domainName = [myHTMLString
stringByReplacingRegexPattern:
@"^.*href=[”’]http://(.*)/.*$" withString:@"$1"
caseInsensitive:YES];
What does it do if given:
<a href=“http://1.example.com/”>This is a link</
a> but <a href=“http://2.example.com/”>This is a
link, too.</a>

46. Beware Greedy
Matching
Remember this?
NSString *domainName = [myHTMLString
stringByReplacingRegexPattern:
@"^.*href=[”’]http://(.*)/.*$" withString:@"$1"
caseInsensitive:YES];
What does it do if given:
<a href=“http://1.example.com/”>This is a link</
a> but <a href=“http://2.example.com/”>This is a
link, too.</a>

47. What you meant was:
After ‘http://’ up to but not including the next ‘/’

48. What you meant was:
After ‘http://’ up to but not including the next ‘/’
Which is:
http://([^/][^/]*)/

49. Remember this?
(?<!(=)|(="")|(='))(((http|ftp|
https)://)|(www.))+[w]+(.[w]+)
([w-.@?^=%&amp;:/~+#]*[w-@?
^=%&amp;/~+#])?(?!.*/a>)
Well, Actually, he has n! problems where,
n is the number of hyperlinks in the input string

50. So if you had
<p>Today’s Links:</p>
<UL>
<LI><A HREF=”http://example.com/1”>Link 1</A></LI>
<LI><A HREF=”http://example.com/2”>Link 2</A></LI>
<LI><A HREF=”http://example.com/3”>Link 3</A></LI>
<LI><A HREF=”http://example.com/4”>Link 4</A></LI>
<LI><A HREF=”http://example.com/5”>Link 5</A></LI>
<LI><A HREF=”http://example.com/6”>Link 6</A></LI>
</UL>

51. And tried to use:
(?<!(=)|(="")|(='))(((http|ftp|
https)://)|(www.))+[w]+(.[w]+)
([w-.@?^=%&amp;:/~+#]*[w-@?
^=%&amp;/~+#])?(?!.*/a>)

52. It would have to:
<p>Today’s Links:</p>
<UL>
<LI><A HREF=”http://example.com/1”>Link 1</A></LI>
<LI><A HREF=”http://example.com/2”>Link 2</A></LI>
<LI><A HREF=”http://example.com/3”>Link 3</A></LI>
<LI><A HREF=”http://example.com/4”>Link 4</A></LI>
<LI><A HREF=”http://example.com/5”>Link 5</A></LI>
<LI><A HREF=”http://example.com/6”>Link 6</A></LI>
</UL>

53. And then:
<p>Today’s Links:</p>
<UL>
<LI><A HREF=”http://example.com/1”>Link 1</A></LI>
<LI><A HREF=”http://example.com/2”>Link 2</A></LI>
<LI><A HREF=”http://example.com/3”>Link 3</A></LI>
<LI><A HREF=”http://example.com/4”>Link 4</A></LI>
<LI><A HREF=”http://example.com/5”>Link 5</A></LI>
<LI><A HREF=”http://example.com/6”>Link 6</A></LI>
</UL>

54. And then:
<p>Today’s Links:</p>
<UL>
<LI><A HREF=”http://example.com/1”>Link 1</A></LI>
<LI><A HREF=”http://example.com/2”>Link 2</A></LI>
<LI><A HREF=”http://example.com/3”>Link 3</A></LI>
<LI><A HREF=”http://example.com/4”>Link 4</A></LI>
<LI><A HREF=”http://example.com/5”>Link 5</A></LI>
<LI><A HREF=”http://example.com/6”>Link 6</A></LI>
</UL>

55. And then:
<p>Today’s Links:</p>
<UL>
<LI><A HREF=”http://example.com/1”>Link 1</A></LI>
<LI><A HREF=”http://example.com/2”>Link 2</A></LI>
<LI><A HREF=”http://example.com/3”>Link 3</A></LI>
<LI><A HREF=”http://example.com/4”>Link 4</A></LI>
<LI><A HREF=”http://example.com/5”>Link 5</A></LI>
<LI><A HREF=”http://example.com/6”>Link 6</A></LI>
</UL>

56. And then:
<p>Today’s Links:</p>
<UL>
<LI><A HREF=”http://example.com/1”>Link 1</A></LI>
<LI><A HREF=”http://example.com/2”>Link 2</A></LI>
<LI><A HREF=”http://example.com/3”>Link 3</A></LI>
<LI><A HREF=”http://example.com/4”>Link 4</A></LI>
<LI><A HREF=”http://example.com/5”>Link 5</A></LI>
<LI><A HREF=”http://example.com/6”>Link 6</A></LI>
</UL>

57. And then:
<p>Today’s Links:</p>
<UL>
<LI><A HREF=”http://example.com/1”>Link 1</A></LI>
<LI><A HREF=”http://example.com/2”>Link 2</A></LI>
<LI><A HREF=”http://example.com/3”>Link 3</A></LI>
<LI><A HREF=”http://example.com/4”>Link 4</A></LI>
<LI><A HREF=”http://example.com/5”>Link 5</A></LI>
<LI><A HREF=”http://example.com/6”>Link 6</A></LI>
</UL>

58. And then:
<p>Today’s Links:</p>
<UL>
<LI><A HREF=”http://example.com/1”>Link 1</A></LI>
<LI><A HREF=”http://example.com/2”>Link 2</A></LI>
<LI><A HREF=”http://example.com/3”>Link 3</A></LI>
<LI><A HREF=”http://example.com/4”>Link 4</A></LI>
<LI><A HREF=”http://example.com/5”>Link 5</A></LI>
<LI><A HREF=”http://example.com/6”>Link 6</A></LI>
</UL>

59. And so on:
<p>Today’s Links:</p>
<UL>
<LI><A HREF=”http://example.com/1”>Link 1</A></LI>
<LI><A HREF=”http://example.com/2”>Link 2</A></LI>
<LI><A HREF=”http://example.com/3”>Link 3</A></LI>
<LI><A HREF=”http://example.com/4”>Link 4</A></LI>
<LI><A HREF=”http://example.com/5”>Link 5</A></LI>
<LI><A HREF=”http://example.com/6”>Link 6</A></LI>
</UL>

60. But what are they
good for?
Encoding/decoding metadata from image file
names.

61. But what are they
good for?
Encoding/decoding metadata from image file
names.
Renaming files on the command line (@2x?)

62. But what are they
good for?
Encoding/decoding metadata from image file
names.
Renaming files on the command line (@2x?)
Grabbing the user’s first name from a Full
Name string (careful of Locales*)
*See http://www.kalzumeus.com/2010/06/17/falsehoods-programmers-believe-about-names/

63. But what are they
good for?
Encoding/decoding metadata from image file
names.
Renaming files on the command line (@2x?)
Grabbing the user’s first name from a Full
Name string (careful of Locales)
Stripping crap I don’t want out of user input
(trailing spaces, anyone?)

64. But what are they
good for?
Encoding/decoding metadata from image file
names.
Renaming files on the command line (@2x?)
Grabbing the user’s first name from a Full
Name string (careful of Locales)
Stripping crap I don’t want out of user input
(trailing spaces, anyone?)
//.*[.* *release *] *;

65. Questions?
CarlB@PDAgent.com
@CarlAllenBrown
www.escortmissions.com (Blog)
www.PDAgent.com (Company)
https://github.com/carlbrown
http://www.slideshare.net/carlbrown