Poznań Ruby User Group presentation, which took place 31.03.2022. Regex is used for google analytics and much more! We dig into ruby amenities of default Regexp class, but go beyond it (so it's for more advanced developers): - explaining how backtracking and deterministic finate automatom works. - why to reach RE2 library from google? or regular expressions at all

1. How to check
valid email?
Not only in Ruby
brought to you by
Piotr Wasiak
Find using RegEx(p?)

2. Agenda
2
1. RegEx overview
2. Recommendations
3. Ruby quirks / amenities
4. Tools / Resources
5. Advanced RE(2)

3. Who am I?
Piotr Wasiak
Ruby, Rails developer
Current PRUG organiser
3
Interests:
● climbing, hiking, squash
● contract bridge, chess
● ruby, programming, crypto

4. Regular Expression
is a character sequence, that defines a search pattern
The purpose is:
● validate the string by the pattern
● get parts of the content (e.g. find or find_and_replace in text editors)
4

5. RegEx history
● Concept of language arose in the 1950s
● Different syntaxes (1980+):
○ POSIX (Basic - or Extended Regular Expressions)
○ Perl (influenced/imported to other languages as PCRE 1997, PCRE2 2015)
5

6. RegEx as a state machine
6
Statement validation: /(?<name>ADAM|PIOTR)s?[=><]{1,2}s*"(?:PIENIĄDZ|KUKU)"/g

7. Basics
7

8. Find RegEx
In replace we can use
matched whole
phrase or groups.
Group number is
ordered by starting
bracket index and is
limited to 1 - 9
8

9. Valid email (1/3)
Rails popular gem solution:
9

10. Valid email (2/3)
10
Email validation:
/(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|"
(?:[x01-x08x0bx0cx0e-x1fx21x23-x5bx5d-x7f]|[x01-x09x0bx0c
x0e-x7f])*")@(?:(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?.)+[a-z0-9]
(?:[a-z0-9-]*[a-z0-9])?|[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?).){3}
(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-z0-9-]*[a-z0-9]:(?:[x01-x08x0
bx0cx0e-x1fx21-x5ax5d-x7f]|[x01-x09x0bx0cx0e-x7f])+)])/g

11. Valid email (3/3)
11
Email validation:
/(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|"
(?:[x01-x08x0bx0cx0e-x1fx21x23-x5bx5d-x7f]|[x01-x09x0bx0c
x0e-x7f])*")@(?:(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?.)+[a-z0-9]
(?:[a-z0-9-]*[a-z0-9])?|[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?).){3}
(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-z0-9-]*[a-z0-9]:(?:[x01-x08x0
bx0cx0e-x1fx21-x5ax5d-x7f]|[x01-x09x0bx0cx0e-x7f])+)])/g

12. 12
2. Recommendations

13. original_regexp =
%r{(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|"(?:[x01-x08x0bx0cx0e-x1f!#-x5b]-x7f]|[x01-x09x0bx0cx0e-x7f])*")@(?:(?:[[:alnum:]](?:[a-z0-9
-]*[[:alnum:]])?.)+[[:alnum:]](?:[a-z0-9-]*[[:alnum:]])?|[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?).){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-z0-9-]*[[:alnum:]]:(?:[x01-x08x0bx
0cx0e-x1f!-Z]-x7f]|[x01-x09x0bx0cx0e-x7f])+)])}
alnum_with_hypen = /[a-z0-9-]/.source # posix alternative /[-[:alnum:]]/
ip_number_type = /25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?/.source
common_parts = /[x01-x08x0bx0cx0e-x1f]-x7f]/.source
username_without_backslash_prepended_set = /[#{common_parts}!#-x5b]/.source
domain_port_unescaped_set = /[#{common_parts}!-Z]/.source
domain_port_escaped_chars_set = /[#{common_parts}x0e-x7f]/.source
non_ending_chars = %r{[a-z0-9!#$%&'*+/=?^_`{|}~-]+}.source
final_with_variables =
/(?:#{non_ending_chars}(?:.#{non_ending_chars})*|"(?:#{username_without_backslash
_prepended_set}|#{domain_port_escaped_chars_set})*")@(?:(?:[[:alnum:]](?:#{alnum
_with_hypen}*[[:alnum:]])?.)+[[:alnum:]](?:#{alnum_with_hypen}*[[:alnum:]])?|[(?
:(?:#{ip_number_type}).){3}(?:#{ip_number_type}|#{alnum_with_hypen}*[[:alnum:]]:(
?:#{domain_port_unescaped_set}|#{domain_port_escaped_chars_set})+)])/
13
Simplify valid email

14. original_regexp =
%r{(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|"(?:[x01-x08x0bx0cx0e-x1f!#-x5b]-x7f]|[x01-x09x0bx0cx0e-x7f])*")@(?:(?:[[:alnum:]](?:[a-z0-9
-]*[[:alnum:]])?.)+[[:alnum:]](?:[a-z0-9-]*[[:alnum:]])?|[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?).){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-z0-9-]*[[:alnum:]]:(?:[x01-x08x0bx
0cx0e-x1f!-Z]-x7f]|[x01-x09x0bx0cx0e-x7f])+)])}
alnum_with_hypen = /[a-z0-9-]/.source # posix alternative /[-[:alnum:]]/
ip_number_type = /25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?/.source
ascii_wo_tabs_cr_nl = /[[:ascii:]&&[^x09-x0ax0d]]/.source
domain_port_escaped_chars_set = /[#{ascii_wo_tabs_cr_nl}x09x20"]/.source
domain_port_unescaped_set = /[#{ascii_wo_tabs_cr_nl}&&[^x20]]/.source
username = /[#{domain_port_unescaped_set}&&[^"]]/.source
non_ending_chars = %r{[a-z0-9!#$%&'*+/=?^_`{|}~-]+}.source
final_with_variables =
/(?:#{non_ending_chars}(?:.#{non_ending_chars})*|"(?:#{username}|#{domain_port_
escaped_chars_set})*")@(?:(?:[[:alnum:]](?:#{alnum_with_hypen}*[[:alnum:]])?.)+[[
:alnum:]](?:#{alnum_with_hypen}*[[:alnum:]])?|[(?:(?:#{ip_number_type}).){3}(?:#
{ip_number_type}|#{alnum_with_hypen}*[[:alnum:]]:(?:#{domain_port_unescaped_set}|
#{domain_port_escaped_chars_set})+)])/
14
Simplify valid email (more ruby version)

15. original_regexp = %r{ # there is no heredoc for regexp
(?: # strings with some special chars, but not ending with .
[a-z0-9!#$%&'*+/=?^_`{|}~-]+
(?:
.[a-z0-9!#$%&'*+/=?^_`{|}~-]+
)*
|
"
(?: # special chars enquoted
[x01-x08x0bx0cx0e-x1f!#-x5b]-x7f]
|
# prepended with backslash, here escaped
[x01-x09x0bx0cx0e-x7f] # more special chars
)*
" # closing quote
)
@ # the most crucial ampersand
(?: # domain regexp
(?: # at least one subdomain joined and finished with .
[[:alnum:]]
(?:
[a-z0-9-]* # subdomain can have many alphanumeric or - inside
[[:alnum:]] # subdomain have to finish with alphanumeric char
)?
. # dot separator
)+
[[:alnum:]] # domain have to start with alphanumeric char
(?:
[a-z0-9-]* # domain can have many alphanumeric or - inside
[[:alnum:]] # domain have to finish with alphanumeric char
)? 15
/x comments mode
| # or direct ip implementation or 3 numbers
with . suffix and some special usecases
[ # enquoted with square brackets
(?:
(?: # numbers are quite complex in RegEx
25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]? #
0-255
). # . suffix
){3} # 3 times
(?:
25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]? # 0-255
| # or 3 numbers with . suffix and some
special usecases
[a-z0-9-]* # alnums also starting with -
[[:alnum:]] # finishing without -
:
(?:
[x01-x08x0bx0cx0e-x1f!-Z]-x7f] #
many chars
|
# more ansii chars prefixed with
backslash
[x01-x09x0bx0cx0e-x7f]
)+
)
] # closing square bracket
)
}x # switch to treat spaces/new lines and `# `
suffix as comments

16. Ruby simply string methods are faster and more meaningful:
● .start_with? / .end_with?
● .include?(‘some substring’)
● .chomp
● .strip
● .lines
● .split(‘ ’) # without regexp
● .tr(‘from chars’, ‘1-9’)
16
Do not overuse regular expression (1/2)

17. Libraries and gems for common concepts:
● URI(url)
+ .host / .path / .query / .fragment
● File(path_to_file)
+ .dirname / .basename / .extname
● Nokogiri::HTML(
open('https://nokogiri.org/’)
)
17
Do not overuse regular expression (2/2)

18. Do not use REGEX as language parser
Programming languages depend more on language nodes/tree.
There will be always a problem with some exceptions, different coding
styles
In Ruby we need to use Ripper or other tools to decompose Ruby code
into pieces
Markup languages can be parsed by e.g. Nokogiri, Ox, Oj gems easier
and more secure
18

19. Clear RegEx
● extract common parts in alternation
● put more likely to appear words in the front of alternation
● use comments and whitespace with /x modifier
● give a name for captured groups, use also non-captured
● split code to smaller logical pieces
● lint code with ruby -w for warnings
19

20. 3. Ruby quirks / flavor
20

21. mix ? Interpolation of RegEx
MULTILINE
IGNORECASE
EXTENDED
21

22. Joke
Scrabble: what is a longest word from combined RE switch letters?
22
I M N O X

23. Joke
Scrabble: what is a longest word from combined RE switch letters?
23
I M N O X

24. - in general "dot matches at line breaks mode" is turn on with s flag
instead of ruby m flag
- In Ruby, ^ and $ always match on every line.
If you want to specify the beginning of the string, use A.
For the very end of the string, use z (or Z including final line break).
Quirks in Ruby RegEx engine (1/3)
24

25. Quirks in Ruby RegEx engine (2/3)
Ruby does not allow
● look-ahead
● negative look-behind
inside a look-behind, such as:
25

26. - Intersection […&&[…]]
- Subtraction […&&[^…]]
26
Quirks in Ruby RegEx engine (3/3)
Character classes operators

27. Ruby amenities (1/3)
27

28. Ruby amenities (2/3)
28

29. Ruby amenities (3/3)
29

30. 4. Tools / Resources
30

31. Tools / Websites
● regex101.com/
nicest editor, explanation on hover, cheatset, performance analysis
● www.debuggex.com/ visualized graphs with cheat-set
● Visualization plugins for Visual Studio Code
● rubocop and rubocop-performance have some rules for regex
● rubular.com/ check if RegEx works in Ruby 2.5. Other with 2.1
● rubyapi.org/3.1/o/regexp good Ruby docs
31

32. 32

33. 5. Advanced RE(2)
33

34. Backtracking
problem
34
/d-d+$/g

35. Catastrophic backtracking case /a?n
an
=~ an
/
35

36. “Most modern engines are regex-directed because this is the only way to
implement useful features such as lazy quantifiers and backreferences;
and atomic grouping and possessive quantifiers that give extra control
to backtracking.”
PCRE like solutions
36

37. 37

38. 38

39. Back to Finite Automaton - (D/N) FA
39
/abb*a/

40. RegEx to Deterministic Finite Automaton
What RegEx is it?
40

41. RegEx to Deterministic Finite Automaton
/(0|1)*1/ matches: [ 1010101, 1, 10101]
41

42. RegEx to Deterministic Finite Automaton
/(0|1)*1/
42

43. RegEx to Deterministic Finite Automaton
/(0|1)*1/
43

44. RE2
PCRE2
44

45. Sources
● devopedia.org/regex-engines
● patshaughnessy.net/2012/4/3/ (...) rubys-regular-expression-algorithm
● github.com/google/re2/wiki/Syntax
● optimized re2 called hyperscan
● wiki/Determinizacja_automatu_skonczonego
● regular-expressions.info/refrepeat.html
● rexegg.com/regex-optimizations.html
45

46. Thanks for listening
What’s your question?
46