java - 简单英语的正则表达式

Question

我正在处理一个新的 Java 项目，因此我正在阅读已经存在的代码。在代码的一个非常重要的部分，如果发现以下正则表达式，我无法真正知道他们在做什么。任何人都可以用简单的英语解释他们做什么？？

1)

 [^,]*|.+(,).+

2)

(\()?\d+(?(1)\))

Answer 1

下次需要解释正则表达式时，可以使用以下 explain.pl来自 Rick Measham 的服务:

Regex: [^,]*|.+(,).+

NODE                     EXPLANATION
--------------------------------------------------------------------------------
  [^,]*                    any character except: ',' (0 or more times
                           (matching the most amount possible))
--------------------------------------------------------------------------------
 |                        OR
--------------------------------------------------------------------------------
  .+                       any character except \n (1 or more times
                           (matching the most amount possible))
--------------------------------------------------------------------------------
  (                        group and capture to \1:
--------------------------------------------------------------------------------
    ,                        ','
--------------------------------------------------------------------------------
  )                        end of \1
--------------------------------------------------------------------------------
  .+                       any character except \n (1 or more times
                           (matching the most amount possible))

---

Regex: (\()?\d+(?(1)\))

NODE                     EXPLANATION
--------------------------------------------------------------------------------
  (                        group and capture to \1 (optional
                           (matching the most amount possible)):
--------------------------------------------------------------------------------
    \(                       '('
--------------------------------------------------------------------------------
  )?                       end of \1 (NOTE: because you're using a
                           quantifier on this capture, only the LAST
                           repetition of the captured pattern will be
                           stored in \1)
--------------------------------------------------------------------------------
  \d+                      digits (0-9) (1 or more times (matching
                           the most amount possible))
--------------------------------------------------------------------------------
  (?(1)                    if back-reference \1 matched, then:
--------------------------------------------------------------------------------
    \)                       ')'
--------------------------------------------------------------------------------
   |                        else:
--------------------------------------------------------------------------------
                             succeed
--------------------------------------------------------------------------------
  )                        end of conditional on \1

链接

• http://rick.measham.id.au/paste/explain.pl

---

注意条件

JAVA 不支持条件语句!第二种模式的无条件正则表达式类似于:

\d+|\(\d+\)

即数字的非零重复，带或不带括号。

链接

• regular-expressions.info/If-then-else conditionals

  Conditionals are supported by the JGsoft engine, Perl, PCRE and the .NET framework.

---

深度模式

这是第一个模式的测试工具

    import java.util.regex.*;
    //...

    Pattern p = Pattern.compile("[^,]*|.+(,).+");
    String[] tests = {
        "",       // [] is a match with no commas
        "abc",    // [abc] is a match with no commas
        ",abc",   // [,abc] is not a match
        "abc,",   // [abc,] is not a match
        "ab,c",   // [ab,c] is a match with separating comma
        "ab,c,",  // [ab,c,] is a match with separating comma
        ",",      // [,] is not a match
        ",,",     // [,,] is not a match
        ",,,",    // [,,,] is a match with separating comma
    };
    for (String test : tests) {
        Matcher m = p.matcher(test);
        System.out.format("[%s] is %s %n", test,
            !m.matches() ? "not a match"
            : m.group(1) != null
               ? "a match with separating comma"
               : "a match with no commas"
        );
    }

结论

• 要匹配，字符串必须属于以下两种情况之一:
  • 不包含逗号(可能是空字符串)
  • 包含分隔两个非空字符串的逗号
• 在匹配上，\1可以用来区分这两种情况

这里是第二种模式的类似测试工具，在不使用条件(Java 不支持)的情况下重写:

    Pattern p = Pattern.compile("\\d+|(\\()\\d+\\)");
    String[] tests = {
        "",       // [] is not a match
        "0",      // [0] is a match without parenthesis
        "(0)",    // [(0)] is a match with surrounding parenthesis
        "007",    // [007] is a match without parenthesis
        "(007)",  // [(007)] is a match with surrounding parenthesis
        "(007",   // [(007] is not a match
        "007)",   // [007)] is not a match
        "-1",     // [-1] is not a match
    };
    for (String test : tests) {
        Matcher m = p.matcher(test);
        System.out.format("[%s] is %s %n", test,
            !m.matches() ? "not a match"
            : m.group(1) != null
               ? "a match with surrounding parenthesis"
               : "a match without parenthesis"
        );
    }

如前所述，这匹配非零数字，可能被括号括起来(\1 区分两者)。