Not all regular expression engines are created equal. They all have differences and nuances. The .NET regex engine has some amazing advanced features that make it one of the most powerful implementations in existence today. Not all problems should be solved with regular expressions, but after this talk you will have to think harder to find one that can't.

1. Advanced Regular
Expressions in .NET
Patrick Delancy

2. NOTICE!!!
This slide deck has been adapted from a
presentation that was intended to be given live,
in person…. like with a real person in front of
real people. You know… breathing the same air
and all that.
The key points have been transcribed onto
separate slides, so you still get some benefit
from reading through it all, but you are still
missing out on all of the great stories, witty
banter, hilarious costumes, stunning arias … or
something like that.
If you REALLY want to get the most out of this
presentation, go to patrickdelancy.com and ask
him to come give it to your group!

3. This presentation will help
you understand what Regex
is capable of.

4. Don’t bother trying to
memorize the syntax, just
remember the concepts.

5. Then you can make a more
intelligent decision about
when you should and
should not use Regex.

6. Common
Features
...but not ubiquitous
● Non-capturing groups
● Look ahead
● Look behind
● Free-spacing

7. Non-Capturing Groups
^(.*)(@)(.*)$
email@ddress.com
email@ddress.com
[1] = email
[2] = @
[3] = ddress.com
^(.*)(?:@)(.*)$
email@ddress.com
email@ddress.com
[1] = email
[2] = ddress.com

8. Look Ahead
bw+(?=.) # match the word at end of each sentence
# but don’t capture the period.
See Dick. See Jane. See Dick and Jane run.
Dick
Jane
run

9. Look Behind
(?<=b19)d{2}b # match all years in the 1900’s
# capturing only the 2-digit year
1842 1902 1776 1985 2003 1999
02
85
99

10. Free Spacing (Ignore Pattern Whitespace)
new Regex(
@”
b[^@]+ # pattern can now span multiple lines
@
[^b]+b # and include white space for readability
”, RegexOptions.IgnorePatternWhitespace);

11. Less-Common
Features
...in more advanced engines
● Named Captures
● Comments
● Inline Directives
● Conditional Alternation
● Atomic Groups
● Compiled Patterns
● Unicode Categories and
Named Character Blocks

12. Named Captures
^(?<name>.*)(?:@)(?<domain>.*)$
email@ddress.com
email@ddress.com
[name] = email
[domain] = ddress.com

13. Comments
^.*@.*$ # comment to the end of the line
^.*@(?# this is an inline comment).*$

14. Inline Directives
John the (?ix) (?: wiser | better and greater | privy )
John the Wiser, John the BetterAndGreater, john the privy,
John the Better and Greater
John the Wiser
John the BetterAndGreater

15. ^Type:(?:(?<ssn>SSN)|(?<eid>EID)), ID:(?(ssn)d{3}-d{2}-
d{4}|[-d]+)$
Type:SSN, ID:352-23-4567
Type:EID, ID:35-2234567
Type:SSN, ID:35-2234567
Type:EID, ID:???
Conditional Alternation

16. b(in|integer|insert)b
integer
integers
in
insert
Atomic Grouping / Possessive Quantifiers
b(?>in|integer|insert)b
integer
integers
in
insert

17. var pattern
= new Regex(@”a+h+!+”);
return
pattern.IsMatch(value);
Compiled Patterns
var pattern = @”a+h+!+”;
return Regex
.IsMatch(pattern, value);

18. b(?:p{IsGreek}+s?)+p{Pd}s(?>p{IsBasicLatin}+s?)+
Κατα Μαθθαίον - The Gospel of Matthew
Named Character Blocks & Unicode Groups

19. Unique Features
...in the .NET RegEx engine
● Balancing Groups
● Character Class Subtraction
● Explicit Capture Only

20. ^(?:[^{}]|(?<open>{)|(?<-open>}))*(?(open)(?!))$
{ if (true) { return “A”; } else { return “B”; } }
{ if (true) { return “A”; } else { return “B”; }
Balancing Groups

21. [0-9-[1-8]]
0123456789
[0-9-[1-8-[2-7]]]
0123456789
Character Class Subtraction
[w-[aeiou]]
Lazy dog, quick fox, blah,
blah, blah.

22. ^(?<name>[^@+]+(+[^+]+)?)@
(?<domain>(w+).(com|net|org))$
e+mail@ddress.com
e+mail@ddress.com
[name] = e+mail
[2] = +mail
[domain] = ddress.com
[4] = ddress
[5] = com
Explicit Capture Only
(?n)^(?<name>[^@+]+(+[^+]+)?)@
(?<domain>(w+).(com|net|org))$
e+mail@ddress.com
e+mail@ddress.com
[name] = e+mail
[domain] = ddress.com

23. Patrick Delancy
patrickdelancy.com
This Presentation:
patrickdelancy.com/presentations/...
@patrickdelancy
linkedin.com/in/patrickdelancy
google.com/+patrickdelancy

24. Some Additional Resources
• https://en.wikipedia.org/wiki/Comparison_of_regular_expression_engines - This is a little outdated,
but still a good overview of how Regex implementations vary.
• https://msdn.microsoft.com/en-us/library/20bw873z(v=vs.110).aspx#SupportedNamedBlocks –
Here is a reference of all of the named Unicode blocks that .NET supports in Regex. Linked here
because I told you I would : )
• http://www.regular-expressions.info/refflavors.html - This is a very comprehensive reference for
many common Regex engines. Some content may be out of date as new versions of each platform
are released.
• http://www.regexplanet.com/ - An online pattern tester. Not the best interface, but very capable and
has some nice features.