修改 Pattern代碼使 Java 正則表達式的group名稱支持下划線 '_'

為什么

由於工作是做數據ETL的，很多時候會使用到正則對數據進行提取，但是java的正則中的groupname不支持'_'，官方的文檔中是這樣的:

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，只支持大寫字母A-Z、小寫字母a-z、數字0-9

查找源代碼

在java.util.regex.Pattern類的以下源碼中(jdk1.8.141是2789行)有下面這個方法:

    /**
     * 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，源代碼中對groupname的提取是一個while循環，當讀取到的字符是小寫字母(ASCII.isLower)、大寫字母(ASCII.isUpper)、數字(ASCII.isDigit)的時候，會把這個字符添加到StringBuilder中，然后讀取下個字符，知道不滿足這個條件。

修改源代碼

好，現在知道是這個原因了，怎么進行修改呢？
有很多人說不要修改大神寫的代碼，但是沒辦法。
由於不支持'_', 給工作帶來挺多其它麻煩的，比如數據庫中的字段名有'_'，如果正則組不支持下划線的話，就需要一個正則組名和列名的映射關系，或者不用正則組名，使用正則組下標0,1,2...來映射。比較繁瑣。
修改其實很簡單，由於Pattern這個類在源代碼中定義為final的，沒法直接繼承然后overwrite這個方法，就只能在自己的項目下新建一個regex包，將java.util.regex包的類都copy出來，總共是6個

修改Pattern的上述方法，'_'這個字符在ASCII中是95，所以添加一個判斷就可以了:

    private String groupname(int ch) {
        StringBuilder sb = new StringBuilder();
        sb.append(Character.toChars(ch));
        //TODO 增加了ch==95這個條件來支持正則組名支持下划線('_')，
        //源碼為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();
    }

這樣就可以使用我們自己Pattern類了，最后成功運行

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()){
            //打印出來是"worker_10"
            System.out.println(matcher.group("my_name"));
        }
    }
}

最后，這個源碼值改了一小部分，但是卻讓工作輕松了
當然，這樣改是否會影響到其它東西需要時間的檢驗。