Regexen

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# Regexen

.b[[@abelar_s](https://twitter.com/abelar_s) -
[@ParisRB](https://twitter.com/parisrb) -
[@RailsGirlsParis](https://twitter.com/railsgirlsparis)]
 
[maitre-du-monde.fr](maitre-du-monde.fr)

.contrib[[Contribute to this talk on GitHub!](https://github.com/abelards/abelards.github.com/edit/master/talks/regexen.html)]

???

---
# A long long time ago

Old UNIX tools are great.<br />
Tools for more elegant times.<br />
I want to match things.

`"Bonjour" =~ /jour/ # => 3`
`"Bonsoir".matches("jour") # => nil`

```ruby
  r = /#{first_part}@#{domains}\.#{tld}/
  emails.reject {|e| e !~ r }
```

---
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# The Phantom Menace

Matching characters:
- Unix Glob: `_` `*`
- SQL LIKE: `?` `%` 
- Reg Exp: `.` `.*`

---
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# Attack of the Clones

We found the previous character N times:

- 0-1 `/Reg(ular)?/`
- 1-n `/expres+ion/`
- 0-n `/regex.*magic/`

---
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# Revenge of the Characters

French postcodes around Paris:

```ruby
/(91|92|93|94).../ # strings' OR # accepts 94xxx
/9[1234]\d\d\d/ # chars' OR # accepts numbers
/9[1-4]\d{3}/ # counting
```

Match over-enthusiastic expressions:
```ruby
min = /yolooo*/, /yolooo+/, /yolo{3,}/
max = /great!{,5}/
boundaries = /lo{3,5}l/
```

---
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# A new hope

case `/ReGeXp/i`

Multiline matching is error-prone:
```
# start/end of line/string
/^part$/ =~ %Q[in
part
icular]
/p.*ar/m =~ %Q[pec
uliar]
/\Apart\Z/ !~ "particle"
```

RUBY ONLY: `\z` means EOS, `\Z` EOS + chomp

---
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# The Group Strikes Back

- `"banana" =~ /b(.).\1.\1/`
- `# "bonobo", "bikini"`

```ruby
str = "You are strong"
str.gsub(/(\w*) (\w*) (\w*)/, "\\3, \\1 \\2")
# => "strong, You are"
```

---
# The Group Strikes Back

Ruby allow you to get that in the MatchData object

```ruby
title = "I am your father"
title =~ /am (\w*) (\w*)/
result = Regexp.last_match
# => #<MatchData "am your father" 1:"your" 2:"father"> 
```

---
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# Return of the $

```
m = /s(\w{2}).*(c)/.match('haystack')
m == Regexp.last_match
#=> #<MatchData "stac" 1:"ta" 2:"c">

$& == m[0] # => "stac"
$` == m.pre_match # => "hay"
$' == m.post_match #=> "k"
$1 == m[1] #=> "ta"
$3 == m[3] #=> nil
$+ == m[-1] #=> "c"
```

---
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# The RegExp awakens

```
/(?<foo>.)(?<bar>.)/.named_captures
#=> {"foo"=>[1], "bar"=>[2]}

/(?<foo>.)(?<bar>.)(?<baz>.)/.names
#=> ["foo", "bar", "baz"]
```

---
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# Rogue \\1

```ruby
s = "[Changing [brackets]] [into parentheses]"
s.gsub(/\[(.*?)\]/, "(\\1)")
# => "(Changing [brackets)] (into parentheses)"
```

`.*` `.*?` (Greed[oy])

---
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# The Last Jedi

* basic : .* and  \? \+
* `-e`xtended : ./()?+

.info[
Regular expressions are a Type-3 Grammar in Chomsky's classification.<br />
They are, can be run as, and can model automata.]

`/!\` DO NOT PARSE HTML WITH REGEXP!

---
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# Classes of the Old Republic

```
 /\w\W/ # word [a-zA-Z0-9_] & non-\w
 /\s\S/ # whitespace [ \t\r\n\f] & non-\S

/[[:xdigit:]]/ # POSIX [0-9a-fA-F]
 /\p{Alnum}/ # Unicode classes
 /\p{Ll}/ # Unicode General Category
 # 'Letter: Lowercase'
```

---
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# Questions?

.b[[@abelar_s](https://twitter.com/abelar_s) -
[@ParisRB](https://twitter.com/parisrb) -
[@RailsGirlsParis](https://twitter.com/railsgirlsparis)]
 
[maitre-du-monde.fr](maitre-du-monde.fr)

.contrib[[Contribute to this talk on GitHub!](https://github.com/abelards/abelards.github.com/edit/master/talks/regexen.html)]

---
class: center, middle, happy
# Thanks!

.b[[@abelar_s](https://twitter.com/abelar_s) -
[@ParisRB](https://twitter.com/parisrb) -
[@RailsGirlsParis](https://twitter.com/railsgirlsparis)]
 
[maitre-du-monde.fr](maitre-du-monde.fr)

.contrib[[Contribute to this talk on GitHub!](https://github.com/abelards/abelards.github.com/edit/master/talks/regexen.html)]