oracle 正则表达式中的运算符优先级

Operator precedence in regular expressions

提问by Kenny

What is the default operator precedence in Oracle's regular expressions when they don't contain parentheses?

当 Oracle 的正则表达式不包含括号时，它们的默认运算符优先级是多少？

For example, given

例如，给定

 H|ha+

would it be evaluated as H|hand then concatenated to aas in ((H|h)a), or would the Hbe alternated with haas in (H|(ha))?

它会被评估为H|h然后连接到aas in ((H|h)a)，还是会H与haas in交替(H|(ha))？

Also, when does the +kick in, etc.?

另外，什么时候+开始，等等？

回答by Thomas Ayoub

Given the Oracle doc:

鉴于Oracle 文档：

Table 4-2 lists the list of metacharacters supported for use in regular expressions passed to SQL regular expression functions and conditions. These metacharacters conform to the POSIX standard; any differences in behavior from the standard are noted in the "Description" column.

表 4-2 列出了支持在传递给 SQL 正则表达式函数和条件的正则表达式中使用的元字符列表。这些元字符符合 POSIX 标准；与标准行为的任何差异都在“描述”列中注明。

And taking a look at the |value in that table:

并查看|该表中的值：

The expression a|b matches character a or character b.

表达式 a|b 匹配字符 a 或字符 b。

Plus taking a look at the POSIX doc:

再加上看一下POSIX 文档：

Operator precedence The order of precedence for of operators is as follows:

1. Collation-related bracket symbols [==] [::] [..]

2. Escaped characters \

3. Character set (bracket expression) []

4. Grouping ()

5. Single-character-ERE duplication * + ? {m,n}

6. Concatenation

7. Anchoring ^$

8. Alternation |

运算符优先级运算符的优先级顺序如下：

1. 与排序规则相关的括号符号 [==] [::] [..]

2. 转义字符 \

3. 字符集（括号表达式）[]

4. 分组 ()

5. 单字符 ERE 重复 * + ? {m,n}

6. 级联

7. 锚定^$

8. 交替|

I would say that H|ha+would be the same as (?:H|ha+).

我会说这H|ha+与(?:H|ha+).

回答by robinCTS

Using capturing groups to demonstrate the order of evaluation, the regex H|ha+is equivalent to the following:

使用捕获组来演示求值顺序，正则表达式H|ha+等效于以下内容：

(H|(h(a+)))

This is because the precedence rules (as seen below) are applied in order from the highest precedence (the lowest numbered) one to the lowest precedence (the highest numbered) one:

这是因为优先级规则（如下所示）是按照从最高优先级（编号最低）到最低优先级（编号最高）的顺序应用的：

• Rule 5 → (a+)The +is grouped with the abecause this operator works on the preceding single character, back-reference, group (a "marked sub-expression" in Oracle parlance), or bracket expression (character class).

• Rule 6 → (h(a+))The his then concatenated with the group in the preceding step.

• Rule 8 → (H|(h(a+)))The His then alternated with the group in the preceding step.

• 规则 5 → (a+)The+与 the 分组，a因为此运算符适用于前面的单个字符、反向引用、组（Oracle 用语中的“标记子表达式”）或括号表达式（字符类）。

• 规则 6 → (h(a+))Theh然后与上一步中的组连接。

• 第8→(H|(h(a+)))将H随后与在前面步骤中的组交替。

---

---

Precedence table from section 9.4.8 of the POSIX docs for regular expressions(there doesn't seem to be an official Oracle table):

正则表达式的 POSIX 文档第9.4.8 节中的优先表（似乎没有官方的 Oracle 表）：

+---+----------------------------------------------------------+
|   |             ERE Precedence (from high to low)            |
+---+----------------------------------------------------------+
| 1 | Collation-related bracket symbols | [==] [::] [..]       |
| 2 | Escaped characters                | \<special character> |
| 3 | Bracket expression                | []                   |
| 4 | Grouping                          | ()                   |
| 5 | Single-character-ERE duplication  | * + ? {m,n}          |
| 6 | Concatenation                     |                      |
| 7 | Anchoring                         | ^ $                  |
| 8 | Alternation                       | |                    |
+---+-----------------------------------+----------------------+

The table above is for Extended Regular Expressions. For Basic Regular Expressions see 9.3.7.

上表用于扩展正则表达式。基本正则表达式见9.3.7。