Snobol4【1】 语言实战：XML【2】 文档类型声明【3】提取
Snobol4 是一种古老的编程语言，最初在1962年由Ralph E. Griswold【4】设计。它以其简洁的语法和强大的字符串处理能力而闻名。尽管Snobol4在现代编程语言中并不常见，但它仍然在一些特定的领域，如文本处理和数据处理中有着独特的应用。本文将探讨如何使用Snobol4语言来提取XML文档的类型声明。
XML 文档类型声明
XML（可扩展标记语言）是一种用于存储和传输数据的标记语言。每个XML文档都包含一个文档类型声明（Document Type Declaration，简称DTD），它定义了文档的结构和元素。DTD通常位于XML文档的第一行，以``开始，后跟``标签。
以下是一个简单的XML文档类型声明的例子：
xml
<#!DOCTYPE root [
]>
在这个例子中，`root`元素包含一个`child`元素，而`child`元素可以包含字符数据。
Snobol4 语言简介
Snobol4 是一种基于字符串处理的编程语言，它使用模式匹配【5】和字符串操作【6】来处理文本数据。Snobol4 的语法相对简单，但功能强大，特别适合于文本处理任务。
Snobol4 的基本语法包括：
- 模式匹配：使用``来定义模式，匹配字符串。
- 变量【7】：使用`$`符号来定义变量。
- 控制结构【8】：包括`if-then-else`、`while`和`for`循环。
- 字符串操作：包括连接、分割、替换等。
Snobol4 提取 XML 文档类型声明
以下是一个使用Snobol4提取XML文档类型声明的示例代码：
```snobol
:in file
:out output
$line = 0
$end = 0
while $line < $end do
$line = $line + 1
$line = $in
if $line = ' ' | $line = '' | $line = 't' then
continue
end
if $line = '' do
$end = $end + 1
$line = $in
end
$line = $in
if $line = '>' then
$output = $output + $line
end
end
end
$end = 0
while $end < $line do
$end = $end + 1
$end = $in
end
$end = 0
while $end < $line do
$end = $end + 1
$end = $in
end
$end = 0
while $end < $line do
$end = $end + 1
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end
$end = 0
while $end < $line do
$end = $end + 1
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end
$end = 0
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end
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end
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end
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end
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end
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end
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end
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end
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