修改 Pattern代码使 Java 正则表达式支持下划线 '_'

为什么

由于工作是做数据ETL的，很多时候会使用到正则对数据进行提取，但是java的正则中的groupname不支持'_'，官方的文档中是这样的:

Group name

A capturing group can also be assigned a "name", a named-capturing group, and then be back-referenced later by the "name". Group names are composed of the following characters. The first character must be a letter.

The uppercase letters 'A' through 'Z' ('\u0041' through '\u005a'),

The lowercase letters 'a' through 'z' ('\u0061' through '\u007a'),

The digits '0' through '9' ('\u0030' through '\u0039'),

A named-capturing group is still numbered as described in Group number.

The captured input associated with a group is always the subsequence that the group most recently matched. If a group is evaluated a second time because of quantification then its previously-captured value, if any, will be retained if the second evaluation fails. Matching the string "aba" against the expression (a(b)?)+, for example, leaves group two set to "b". All captured input is discarded at the beginning of each match.

Groups beginning with (? are either pure, non-capturing groups that do not capture text and do not count towards the group total, or named-capturing group.

可以看到，只支持大写字母A-Z、小写字母a-z、数字0-9

查找源代码

在java.util.regex.Pattern类的以下源码中(jdk1.8.141是2789行)有下面这个方法:

    /**
     * Parses and returns the name of a "named capturing group", the trailing
     * ">" is consumed after parsing.
     */
    private String groupname(int ch) {
        StringBuilder sb = new StringBuilder();
        sb.append(Character.toChars(ch));
        while (ASCII.isLower(ch=read()) || ASCII.isUpper(ch) ||
               ASCII.isDigit(ch)) {
            sb.append(Character.toChars(ch));
        }
        if (sb.length() == 0)
            throw error("named capturing group has 0 length name");
        if (ch != '>')
            throw error("named capturing group is missing trailing '>'");
        return sb.toString();
    }

可以看到，源代码中对groupname的提取是一个while循环，当读取到的字符是小写字母(ASCII.isLower)、大写字母(ASCII.isUpper)、数字(ASCII.isDigit)的时候，会把这个字符添加到StringBuilder中，然后读取下个字符，知道不满足这个条件。

修改源代码

好，现在知道是这个原因了，怎么进行修改呢？

有很多人说不要修改大神写的代码，但是没办法。

由于不支持'_', 给工作带来挺多其它麻烦的，比如数据库中的字段名有'_'，如果正则组不支持下划线的话，就需要一个正则组名和列名的映射关系，或者不用正则组名，使用正则组下标0,1,2...来映射。比较繁琐。

修改其实很简单，由于Pattern这个类在源代码中定义为final的，没法直接继承然后overwrite这个方法，就只能在自己的项目下新建一个regex包，将java.util.regex包的类都copy出来，总共是6个

修改Pattern的上述方法，'_'这个字符在ASCII中是95，所以添加一个判断就可以了:

    private String groupname(int ch) {
        StringBuilder sb = new StringBuilder();
        sb.append(Character.toChars(ch));
        //TODO 增加了ch==95这个条件来支持正则组名支持下划线('_')，
        //源码为java.util.regex.Pattern的2793行
        while (ASCII.isLower(ch=read()) || ASCII.isUpper(ch) ||
               ASCII.isDigit(ch) || ch == 95) {
            sb.append(Character.toChars(ch));
        }
        if (sb.length() == 0)
            throw error("named capturing group has 0 length name");
        if (ch != '>')
            throw error("named capturing group is missing trailing '>'");
        return sb.toString();
    }

这样就可以使用我们自己Pattern类了，最后成功运行

public class MyTest {
    public static void main(String[] args) {
        Pattern pattern = Pattern.compile("\\s\\|\\s(?<my_name>worker_\\d+)\\s\\|");
        Matcher matcher = pattern.matcher("2017-02-14 23:58:04 | worker_10 | [ATMP05]");
        if (matcher.find()){
            //打印出来是"worker_10"
            System.out.println(matcher.group("my_name"));
        }
    }
}

最后，这个源码值改了一小部分，但是却让工作轻松了

当然，这样改是否会影响到其它东西需要时间的检验。