My-Notes-Compilation/Match_n.html.html.html at master · Archive-42/My-Notes-Compilation · GitHub

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang="" xml:lang="">
  <head>
    <meta charset="utf-8" />
    <meta name="generator" content="pandoc" />
    <meta
      name="viewport"
      content="width=device-width, initial-scale=1.0, user-scalable=yes"
    />
    <title>Match n</title>
    <style type="text/css">
      code {
        white-space: pre-wrap;
      }
      span.smallcaps {
        font-variant: small-caps;
      }
      span.underline {
        text-decoration: underline;
      }
      div.column {
        display: inline-block;
        vertical-align: top;
        width: 50%;
      }
    </style>
    <link
      rel="stylesheet"
      href="https://cdn.jsdelivr.net/npm/bootstrap@5.0.0-beta3/dist/css/bootstrap.min.css"
      integrity="sha384-eOJMYsd53ii+scO/bJGFsiCZc+5NDVN2yr8+0RDqr0Ql0h+rP48ckxlpbzKgwra6"
      crossorigin="anonymous"
    />
    <script
      src="https://cdn.jsdelivr.net/npm/bootstrap@5.0.0-beta3/dist/js/bootstrap.min.js"
      integrity="sha384-j0CNLUeiqtyaRmlzUHCPZ+Gy5fQu0dQ6eZ/xAww941Ai1SxSY+0EQqNXNE6DZiVc"
      crossorigin="anonymous"
    ></script>
  </head>
  <body>
    <h1 id="match-n">Match n</h1>
    <p>String length</p>
    <p><code>^.{1,35}$</code></p>
    <p>String that containts a substring ONLY once</p>
    <p><code>^(?:(?!&lt;scr&gt;).)*&lt;scr&gt;(?!.*&lt;scr&gt;).*$</code></p>
    <p>No identical consecutive characters</p>
    <p><code>^(([\w])(?!\2))+$</code></p>
    <p>No more than two identical consecutive characters, etc</p>
    <p><code>^(([\w])\2?(?!\2))+$</code></p>
    <h3
      id="at-least-two-unique-characters-httpsstackoverflow.comquestions5341369regex-for-at-least-two-unique-characters"
    >
      At least two unique characters:
      https://stackoverflow.com/questions/5341369/regex-for-at-least-two-unique-characters/
    </h3>
    <pre><code>^
(?&gt;                 # Possessive group - do not backtrack!
    (.)             # Match a duplicated character
       (?:
         (?=.*\1)       # It can have a duplicate after itself
         |
         (?&lt;=\1.+)      # Or it already had one
       )
    |               # Or, it isn&#39;t a duplicated character at all
    (?&lt;Unique&gt;.)    # Capture it as a unique character.
)+
$
(?&lt;-Unique&gt;){2}     # After we&#39;re done, check there were at least
                    # two unique characters</code></pre>
    <p>
      The idea here is to match duplicated characters and count the rest using
      balancing groups: This is achieved with a possessive group - it makes sure
      the duplicated characters never backtrack, thus the next dot will only
      capture a non-duplicated character.<br />
      In .Net, every capture of a group is added to a stack. (?&lt;-Unique&gt;)
      pops a capture from the stack, and fails if it’s empty. It gives a nice
      way of counting how many captures we’ve had.
    </p>
    <p>Or a simpler solution:</p>
    <pre><code>(.)(?&lt;!\1.+)(?!.*\1).*(.)(?&lt;!\2.+)(?!.*\2)</code></pre>
    <p>To explain it:</p>
    <pre><code>(.)                    # match any character...
(?&lt;!\1.+)              # ...which does not already exist in the input...
(?!.*\1)               # ..and does not exist later on in the input
                       # We have now found one unique character.

.*                     # allow for any number of random characters in the middle

(.)(?&lt;!\2.+)(?!.*\2)   # Find a second unique character,
                       # using the same technique.</code></pre>
  </body>
</html>