Learn regular expression options, assertions

Regular expressions for the last few days , Record it here , If you forget later, you can look back .

Explain regular expressions ：(?is)(?<=([\W\u4e00-\u9fa5A-Za-z0-9].*\n){6})(.*)(?=：\s*?\r\n\s+?)

Knowledge point 1* (?is) Regular expression options

link ： Regular expression option learning

Inline matching pattern , The inline match pattern is usually used instead of enumerating values RegexOptions The specified global match pattern .

Be careful ：

1. (?s) Usually used when matching text with a newline ;
2. (?m) Only regular expressions involving multiple lines “^” and “$” When matching , Only use Multiline Pattern ;
3. The above matching patterns can be combined , such as (?is),(?im);
4. It can be used (?i:exp) perhaps (?i)exp(?-i) To specify the valid range of the match .

summary ：(?is) The meaning of is that the expression on the right ignores case and . Can match each character

Knowledge point 2* (?<=exp)(?=) Zero width assertion 、 Look around

link ：1. Assertion ;2.(exp),(?:exp),(?=exp) understand ;3. Zero width assertion details

With (?=exp) For example , This is called " Zero width is asserting ", There are other articles called " Zero - width forward prediction antecedent assertion ", Name doesn't matter , The description is the same .

" Go ahead " Indicates that the position to be verified is in the front , That is, the position of verification is exp Before the matching content
" just " Express exp Describe the rules , Match the content after the verification position

summary ：
(?<=([\W\u4e00-\u9fa5A-Za-z0-9].\n){6}) Indicates that the matching content is satisfied in front of “([\W\u4e00-\u9fa5A-Za-z0-9].\n){6}” expression ;
(?=：\s*?\r\n\s+?) It means that the matching content meets “：\s*?\r\n\s+?” expression .

Knowledge point 3* () Regular expression groups

link ：Java Regular expression groups

1. We can combine multiple characters into one unit through parentheses . for example ,(ab)
2. Each group in a regular expression has a group number , from 1 Start
3. Matcher Methods in class groupCount() Return and Matcher The number of instances in the associated pattern
4. Group 0 Reference to the entire regular expression and not by groupCount() Method report
5. Each left parenthesis in the regular expression marks the beginning of a new group
6. We can return the reference group number in the regular expression .
   summary ：(.*) Indicates matching multiple arbitrary characters .

Knowledge point 4* Greedy and lazy patterns

link ： .? or .+ What does that mean?

greedy ： Match strings as long as possible
lazy ： Match the shortest possible string

Lazy mode is enabled by simply adding... After repeating metacharacters ? that will do

1. *? Repeat any number of times , But repeat as little as possible
2. +? repeat 1 Times or more , But repeat as little as possible
3. ?? repeat 0 Time or 1 Time , But repeat as little as possible
4. {n,m}? repeat n To m Time , But repeat as little as possible
5. {n,}? repeat n More than once , But repeat as little as possible

Basic knowledge point reference ： regular expression syntax ;java Regular expressions ; Regular expression concise reference