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字符匹配：
- 普通字符：匹配与其自身相等的字符。例如，
a匹配字符"a"。 - 字符类：使用方括号
[]定义一个字符类，匹配方括号中列举的任意字符。例如，[abc]匹配字符"a"、"b"或"c"。 - 范围类：在字符类中使用连字符
-表示范围。例如，[a-z]匹配任意小写字母。 - 反向类：在字符类前加上脱字符
^，表示匹配除了字符类中列举的字符之外的任意字符。例如，[^0-9]匹配除了数字之外的任意字符。
- 普通字符：匹配与其自身相等的字符。例如，
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重复匹配：
*：匹配前面的表达式零次或多次。+：匹配前面的表达式一次或多次。?：匹配前面的表达式零次或一次。{n}：匹配前面的表达式恰好 n 次。{n,}：匹配前面的表达式至少 n 次。{n,m}：匹配前面的表达式至少 n 次，但不超过 m 次。
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特殊字符：
\d：匹配任意数字字符。\w：匹配任意字母、数字或下划线字符。\s：匹配任意空白字符。\b：匹配单词边界。.：匹配除换行符以外的任意字符。
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分组和引用：
()：用于分组，将多个表达式组合为一个整体。(?:)：非捕获分组，用于分组但不捕获匹配的内容。\n：引用前面的分组，n 表示分组的序号。
一份 Cheat Sheet
| Anchor | Description | Example | Valid match | Invalid | ||
|---|---|---|---|---|---|---|
| ^ | start of string or line | ^foam | foam | bath foam | ||
| \A | start of string in any match mode | \Afoam | foam | bath foam | ||
| $ | end of string or line | finish$ | finish | finnish | ||
| \Z | end of string, or char before last new line in any match mode | finish\Z | finish | finnish | ||
| \z | end of string, in any match mode. | |||||
| \G | end of the previous match or the start of the string for the first match | ^(get | set) | \G\w+$ | setValue | seValue |
| \b | word boundary; position between a word character (\w), and a nonword character (\W) | \bis\b | This island is beautiful | This island isn't beautiful | ||
| \B | not-word-boundary. | \Bland | island | peninsula |
| Assertion | Description | Example | Valid match | Invalid |
|---|---|---|---|---|
| (?=...) | positive lookahead | question(?=s) | questions | question |
| (?!...) | negative lookahead | answer(?!s) | answer | answers |
| (?<=...) | positive look-behind | (?<=appl)e | apple | application |
| (?<!...) | negative look-behind | (?<!goo)d | mood | good |
| Char class | Description | Example | Valid match | Invalid |
|---|---|---|---|---|
| [ ] | class definition | [axf] | a, x, f | b |
| [ - ] | class definition range | [a-c] | a, b, c | d |
| [ \ ] | escape inside class | [a-f.] | a, b, . | g |
| [^ ] | Not in class | [^abc] | d, e | a |
| [:class:] | POSIX class | [:alpha:] | string | 0101 |
| . | match any chars except new line | b.ttle | battle, bottle | bttle |
| \s | white space, [\n\r\f\t ] | good\smorning | good morning | good.morning |
| \S | no-white space, [^\n\r\f\t] | good\Smorning | good.morning | good morning |
| \d | digit | \d{2} | 23 | 1a |
| \D | non-digit | \D{3} | foo, bar | fo1 |
| \w | word, [a-z-A-Z0-9_] | \w{4} | v411 | v4.1 |
| \W | non word, [^a-z-A-Z0-9_] | .$%? | .$%? | .ab? |
| Special character | Description | |
|---|---|---|
| general escape | ||
| \n | new line | |
| \r | carriage return | |
| \t | tab | |
| \v | vertical tab | |
| \f | form feed | |
| \a | alarm | |
| [\b] | backspace | |
| \e | escape | |
| \cchar | Ctrl + char(ie:\cc is Ctrl+c) | |
| \ooo | three digit octal (ie: \123) | |
| \xhh | one or two digit hexadecimal (ie: \x10) | |
| \x{hex} | any hexadecimal code (ie: \x{1234}) | |
| \p{xx} | char with unicode property (ie: \p{Arabic} | |
| \P{xx} | char without unicode property |
| Sequence | Description | Example | Valid match | Invalid | ||
|---|---|---|---|---|---|---|
| alternation | apple | orange | apple, orange | melon | ||
| ( ) | subpattern | foot(er | ball) | footer or football | footpath | |
| (?P<name>...) | subpattern, and capture submatch into name | (?P<greeting>hello) | hello | hallo | ||
| (?:...) | subpattern, but does not capture submatch | (?:hello) | hello | hallo | ||
| + | one or more quantifier | ye+ah | yeah, yeeeah | yah | ||
| * | zero or more quantifier | ye*ah | yeeah, yeeeah, yah | yeh | ||
| ? | zero or one quantifier | yes? | yes, ye | yess | ||
| ?? | zero or one, as few times as possible (lazy) | yea??h | yeah | yeaah | ||
| +? | one or more lazy | /<.+?>/g | <P>foo</P> matches only <P> and </P> | |||
| *? | zero or more, lazy | /<.*?>/g | <html> | |||
| {n} | n times exactly | fo{2} | foo | fooo | ||
| {n,m} | from n to m times | go{2,3}d | good,goood | gooood | ||
| {n,} | at least n times | go{2,} | goo, gooo | go | ||
| (?(condition)...) | if-then pattern | (<)?[p](?(1)>) | <p>, p | <p | ||
| (?(condition)... | ...) | if-then-else pattern | `^(?(?=q)que | ans)` | question, answer |
| Pattern modifier | Description |
|---|---|
| g | global match |
| i | case-insensitiv, match both uppercase and lowercase |
| m | multiple lines |
| s | single line (by default) |
| x | ingore whitespace allows comments |
| A | anchored, the pattern is forced to ^ |
| D | dollar end only, a dollar metacharacter matches only at the end |
| S | extra analysis performed, useful for non-anchored patterns |
| U | ungreedy, greedy patterns becomes lazy by default |
| X | additional functionality of PCRE (PCRE extra) |
| J | allow duplicate names for subpatterns |
| u | unicode, pattern and subject strings are treated as UTF-8 |
