The introduction

The most recent comparison between ^ and $and (? : the pattern) and (? =pattern), for example: For a parenthesis (e.g., 1+2*(2+3)), there is now a ready-made static method calculateFormula(String formala) that implements: Enter any of the four ordinary expressions without parentheses and return the result.

Considering the priority of operation, we need to calculate the subformula 2+3 first, and get the calculation result 5 through calculateFormula(String formala). Finally, replace the subformula 2+3 in the original formula 1+2*(2+3) with 5, and get an equation 1+2*5 without parentheses. Finally, calculateFormula(String formala) is called to get the final result.

To complete the above example, the two most important points are:

  1. Gets the subexpression enclosed in parentheses2 + 3(Note the contents without parentheses)
  2. will(1 + 2 * 2 + 3)In the(2 + 3)(Note the parenthesized content) is replaced by5

Let’s try to get 2+3 by using a regular expression

The regular expression is used to get the subexpression inside the parentheses — (? The use of = the pattern)

To get the 2+3 of 1+2*(2+3), you match the part of the expression enclosed by () and remove () using the expression :(? () < = \ \ [^ \ \ (\ \)] * (? = \ \))

(? <=\\() means to locate the position after the symbol (, note that this is positioning!! Instead of matching.

[^ \ \ (\ \)] * on behalf of the match zero or more than (or) characters, this is to avoid nested, such as, ((2 + 3) + 1) + 1, this time not matching to the characters ((2 + 3) + 1), and will match only to (2 + 3), to ensure that the formula is does not include the parentheses.

(? =\\)) represents positioning to a position preceding the symbol)

(? : the pattern) and (? The difference between = the pattern)

(? :pattern) the matching result contains pattern

(? +pattern) the matching result does not contain pattern

In terms of the final matching effect (? There is no difference between:

  • Matching results of regular expressions

Meaning of ^ and $in character matching

For example: 1+2*(2+3), the following is the case:

  1. Regular expressions using the ^ and $symbols are used for global character matching, i.e.(1 + 2 * 2 + 3)Matches the entire content of
  2. A regular expression with the ^ sign is a pair of substrings headed with the original string, which is true for the example above1, 1 + 2, 1 + 2 *. Matches with substrings
  3. A regular expression with the $sign is a pair of substrings that end in the original string, which is true for the example above), 3) + 3). Matches with substrings
  4. A regular expression without the ^ and $symbols matches any subcharacter