跟着 Lua 5.1 官方参考文档学习 Lua (11)

5.4.1 – Patterns

Character Class:

A character class is used to represent a set of characters. The following combinations are allowed in describing a character class:

• x: (where x is not one of the magic characters ^$()%.[]*+-?) represents the character x itself.

• .: (a dot) represents all characters.

• %a: represents all letters.

• %c: represents all control characters.

• %d: represents all digits.

• %l: represents all lowercase letters.

• %p: represents all punctuation characters.

• %s: represents all space characters.

• %u: represents all uppercase letters.

• %w: represents all alphanumeric characters.

• %x: represents all hexadecimal digits.

• %z: represents the character with representation 0.

• %x: (where x is any non-alphanumeric character) represents the character x. This is the standard way to escape the magic characters. Any punctuation character (even the non magic) can be preceded by a ‘%’ when used to represent itself in a pattern.

• [set]: represents the class which is the union of all characters in set. A range of characters can be specified by separating the end characters of the range with a ‘-’. All classes %x described above can also be used as components in set. All other characters in set represent themselves.

  For example, [%w_] (or [_%w]) represents all alphanumeric characters plus the underscore,

  [0-7] represents the octal digits, and [0-7%l%-] represents the octal digits plus the lowercase letters plus the ‘-’ character.

  The interaction between ranges and classes is not defined. Therefore, patterns like [%a-z] or [a-%%] have no meaning.

• [^set]: represents the complement of set, where set is interpreted as above.

For all classes represented by single letters (%a, %c, etc.), the corresponding uppercase letter represents the complement of the class. For instance, %S represents all non-space characters.

The definitions of letter, space, and other character groups depend on the current locale. In particular, the class [a-z] may not be equivalent to %l.

Pattern Item:

A pattern item can be

• a single character class, which matches any single character in the class;
• a single character class followed by ‘*’, which matches 0 or more repetitions of characters in the class. These repetition items will always match the longest possible sequence;
• a single character class followed by ‘+’, which matches 1 or more repetitions of characters in the class. These repetition items will always match the longest possible sequence;
• a single character class followed by ‘-’, which also matches 0 or more repetitions of characters in the class. Unlike ‘*’, these repetition items will always match the shortest possible sequence;
• a single character class followed by ‘?’, which matches 0 or 1 occurrence of a character in the class;
• %n, for n between 1 and 9; such item matches a substring equal to the n-th captured string (see below);
• %bxy, where x and y are two distinct characters; such item matches strings that start with x, end with y, and where the x and y are balanced. This means that, if one reads the string from left to right, counting +1 for an x and -1 for a y, the ending y is the first y where the count reaches 0. For instance, the item %b() matches expressions with balanced parentheses.

Pattern:

A pattern is a sequence of pattern items.

A ‘^’ at the beginning of a pattern anchors the match at the beginning of the subject string.

A ‘$’ at the end of a pattern anchors the match at the end of the subject string.

At other positions, ‘^’ and ‘$’ have no special meaning and represent themselves.

Captures:

A pattern can contain sub-patterns enclosed in parentheses; they describe captures. When a match succeeds, the substrings of the subject string that match captures are stored (captured) for future use. Captures are numbered according to their left parentheses. For instance, in the pattern "(a*(.)%w(%s*))", the part of the string matching "a*(.)%w(%s*)" is stored as the first capture (and therefore has number 1); the character matching “.” is captured with number 2, and the part matching “%s*” has number 3.

As a special case, the empty capture () captures the current string position (a number). 【注意：() 的含义】For instance, if we apply the pattern "()aa()" on the string "flaaap", there will be two captures: 3 and 5.

A pattern cannot contain embedded zeros. Use %z instead.

string.find (s, pattern [, init [, plain]])

Looks for the first match of pattern in the string s. If it finds a match, then find returns the indices of s where this occurrence starts and ends; otherwise, it returns nil. A third, optional numerical argument init specifies where to start the search; its default value is 1 and can be negative. A value of true as a fourth, optional argument plain turns off the pattern matching facilities【plain参数为true，那么就是普通的字符串查找】, so the function does a plain “find substring” operation, with no characters in pattern being considered “magic”. Note that if plain is given, then init must be given as well.

If the pattern has captures, then in a successful match the captured values are also returned, after the two indices.

例子：string.find 的简单使用

s = "hello world"
i, j = string.find(s, "hello")
print(i, j)                    --> 1 5
print(string.sub(s, i, j))     --> hello
print(string.find(s, "world")) --> 7 11
i, j = string.find(s, "l")
print(i, j)                    --> 3 3
print(string.find(s, "lll"))   --> nil

s = "Deadline is 30/05/1999, firm"
date = "%d%d/%d%d/%d%d%d%d"
print(string.sub(s, string.find(s, date))) --> 30/05/1999

补充内容：

The string.find function has an optional third parameter: an index that tells where in the subject string to start the search. This parameter is useful when we want to process all the indices where a given pattern appears: we search for a new match repeatedly, each time starting after the position where we found the previous one. As an example, the following code makes a table with the positions of all newlines in a string:

local t = {} -- table to store the indices
local i = 0
while true do
    i = string.find(s, "\n", i + 1) -- find next newline
    if i == nil then break end
    t[#t + 1] = i
end

For instance, the test

if string.find(s, "^%d") then ...

checks whether the string s starts with a digit, and the test

if string.find(s, "^[+-]?%d+$") then ...

checks whether this string represents an integer number, without other leading
or trailing characters.

string.match (s, pattern [, init])

Looks for the first match of pattern in the string s. If it finds one, then match returns the captures from the pattern; otherwise it returns nil. If pattern specifies no captures, then the whole match is returned. A third, optional numerical argument init specifies where to start the search; its default value is 1 and can be negative.

注意比较 match 函数和 find 函数返回值的不同！

find 函数返回字符串中第一个匹配 pattern 的子字符串的位置和 pattern 中所有的 captures。

match 函数返回字符串中第一个匹配 pattern 的子字符串中所有的 captures。

find 函数比 match 函数多返回第一个匹配 pattern 的子字符串的位置。

补充内容：

We can use captures in the pattern itself. In a pattern, an item like ‘%d’, where d is a single digit, matches only a copy of the d-th capture. As a typical use, suppose you want to find, inside a string, a substring enclosed between single or double quotes. You could try a pattern such as ‘[“’].-[”’]’, that is, a quote followed by anything followed by another quote; but you would have problems with strings like “it’s all right”. To solve this problem, you can capture the first quote and use it to specify the second one:

s = [[then he said: "it’s all right"!]]
q, quotedPart = string.match(s, "([\"’])(.-)%1")
print(quotedPart)     --> it’s all right
print(q)

A similar example is the pattern that matches long strings in Lua:

%[(=*)%[(.-)%]%1%]

It will match an opening square bracket followed by zero or more equal signs, followed by another opening square bracket, followed by anything (the string content), followed by a closing square bracket, followed by the same number of equal signs, followed by another closing square bracket:

p = "%[(=*)%[(.-)%]%1%]"
s = "a = [=[[[ something ]] ]==] ]=]; print(a)"
print(string.match(s, p)) --> = [[ something ]] ]==]

The first capture is the sequence of equal signs (only one in this example); the second is the string content

string.gmatch (s, pattern)

Returns an iterator function that, each time it is called, returns the next captures from pattern over string s. If pattern specifies no captures, then the whole match is produced in each call.

As an example, the following loop

     s = "hello world from Lua"
     for w in string.gmatch(s, "%a+") do
       print(w)
     end

will iterate over all the words from string s, printing one per line. The next example collects all pairs key=value from the given string into a table:

     t = {}
     s = "from=world, to=Lua"
     for k, v in string.gmatch(s, "(%w+)=(%w+)") do
       t[k] = v
     end

For this function, a ‘^’ at the start of a pattern does not work as an anchor, as this would prevent the iteration.

string.gsub (s, pattern, repl [, n])

Returns a copy of s in which all (or the first n, if given) occurrences of the pattern have been replaced by a replacement string specified by repl, which can be a string, a table, or a function. gsub also returns, as its second value, the total number of matches that occurred.

If repl is a string, then its value is used for replacement. The character % works as an escape character: any sequence in repl of the form %n, with n between 1 and 9, stands for the value of the n-th captured substring (see below). The sequence %0 stands for the whole match. The sequence %% stands for a single %.

If repl is a table, then the table is queried for every match, using the first capture as the key; if the pattern specifies no captures, then the whole match is used as the key.

If repl is a function, then this function is called every time a match occurs, with all captured substrings passed as arguments, in order; if the pattern specifies no captures, then the whole match is passed as a sole argument.

If the value returned by the table query or by the function call is a string or a number, then it is used as the replacement string; otherwise, if it is false or nil, then there is no replacement (that is, the original match is kept in the string).

Here are some examples:

     x = string.gsub("hello world", "(%w+)", "%1 %1")
     --> x="hello hello world world"
     
     x = string.gsub("hello world", "%w+", "%0 %0", 1)
     --> x="hello hello world"
     
     x = string.gsub("hello world from Lua", "(%w+)%s*(%w+)", "%2 %1")
     --> x="world hello Lua from"
     
     x = string.gsub("home = $HOME, user = $USER", "%$(%w+)", os.getenv)
     --> x="home = /home/roberto, user = roberto"
     
     x = string.gsub("4+5 = $return 4+5$", "%$(.-)%$", function (s)
           return loadstring(s)()
         end)
     --> x="4+5 = 9"
     
     local t = {name="lua", version="5.1"}
     x = string.gsub("$name-$version.tar.gz", "%$(%w+)", t)
     --> x="lua-5.1.tar.gz"

补充内容：

例子：计算包含的元音字母个数

nvow = select(2, string.gsub(text, "[AEIOUaeiou]", ""))

例子 ：'*‘和’-'的区别

test = "int x; /* x */ int y; /* y */"
print(string.gsub(test, "/%*.*%*/", "<COMMENT>")) --> int x; <COMMENT>   1

test = "int x; /* x */ int y; /* y */"
print(string.gsub(test, "/%*.-%*/", "<COMMENT>")) --> int x; <COMMENT> int y; <COMMENT>    2

补充内容：

Another item in a pattern is ‘%b’, which matches balanced strings. Such item is written as ‘%bxy’, where x and y are any two distinct characters; the x acts as an opening character and the y as the closing one. For instance, the pattern ‘%b()’ matches parts of the string that start with a ‘(’ and finish at the respective ‘)’:

s = "a (enclosed (in) parentheses) line"
print(string.gsub(s, "%b()", "")) --> a line

Typically, this pattern is used as ‘%b()’, ‘%b[]’, ‘%b{}’, or ‘%b<>’, but you can use any characters as delimiters.

5.5 – Table Manipulation

This library provides generic functions for table manipulation. It provides all its functions inside the table table.

Most functions in the table library assume that the table represents an array or a list. For these functions, when we talk about the “length” of a table we mean the result of the length operator.

table.concat (table [, sep [, i [, j]]])

Given an array where all elements are strings or numbers, returns table[i]..sep..table[i+1] ··· sep..table[j]. The default value for sep is the empty string, the default for i is 1, and the default for j is the length of the table. If i is greater than j, returns the empty string.

table.insert (table, [pos,] value)

Inserts element value at position pos in table, shifting up other elements to open space, if necessary. The default value for pos is n+1, where n is the length of the table (see §2.5.5), so that a call table.insert(t,x) inserts x at the end of table t.

table.maxn (table)

Returns the largest positive numerical index of the given table, or zero if the table has no positive numerical indices. (To do its job this function does a linear traversal of the whole table.)

table.remove (table [, pos])

Removes from table the element at position pos, shifting down other elements to close the space, if necessary. Returns the value of the removed element. The default value for pos is n, where n is the length of the table, so that a call table.remove(t) removes the last element of table t.

table.sort (table [, comp])

Sorts table elements in a given order, in-place, from table[1] to table[n], where n is the length of the table. If comp is given, then it must be a function that receives two table elements, and returns true when the first is less than the second (so that not comp(a[i+1],a[i]) will be true after the sort). If comp is not given, then the standard Lua operator < is used instead.

The sort algorithm is not stable; that is, elements considered equal by the given order may have their relative positions changed by the sort.

5.6 – Mathematical Functions

This library is an interface to the standard C math library. It provides all its functions inside the table math.

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