REGEXP_SUBSTR延伸SUBSTR函数的功能，让你搜索一个正则表达式模式字符串。 这也类似于REGEXP_INSTR，而是返回子字符串的位置，它返回的子字符串本身。
语法
Oracle数据库中的REGEXP_SUBSTR函数的语法是：
REGEXP_SUBSTR(source_char, pattern [, position [, occurrence [, match_parameter ]]])
参数
source_char
搜索字符串。可以是任意的数据类型char，VARCHAR2，nchar，CLOB，NCLOB类型
pattern
正则表达式
| Value | Description |
| ^ | Matches the beginning of a string. If used with a match_parameter of 'm', it matches the start of a line anywhere within expression. |
| $ | Matches the end of a string. If used with a match_parameter of 'm', it matches the end of a line anywhere within expression. |
| * | 匹配零个或多个. |
| + | 匹配一个或多个出现. |
| ? | 匹配零次或一次出现. |
| . | 匹配任何字符，除了空. |
| | | Used like an "OR" to specify more than one alternative. |
| [ ] | Used to specify a matching list where you are trying to match any one of the characters in the list. |
| [^ ] | Used to specify a nonmatching list where you are trying to match any character except for the ones in the list. |
| ( ) | Used to group expressions as a subexpression. |
| {m} | Matches m times. |
| {m,} | Matches at least m times. |
| {m,n} | Matches at least m times, but no more than n times. |
| \n | n is a number between 1 and 9. Matches the nth subexpression found within ( ) before encountering \n. |
| [..] | Matches one collation element that can be more than one character. |
| [::] | Matches character classes. |
| [==] | Matches equivalence classes. |
| \d | 匹配一个数字字符. |
| \D | 匹配一个非数字字符. |
| \w | 匹配包括下划线的任何单词字符. |
| \W | 匹配任何非单词字符. |
| \s | 匹配任何空白字符，包括空格、制表符、换页符等等. |
| \S | 匹配任何非空白字符. |
| \A | Matches the beginning of a string or matches at the end of a string before a newline character. |
| \Z | Matches at the end of a string. |
| *? | Matches the preceding pattern zero or more occurrences. |
| +? | Matches the preceding pattern one or more occurrences. |
| ?? | Matches the preceding pattern zero or one occurrence. |
| {n}? | Matches the preceding pattern n times. |
| {n,}? | Matches the preceding pattern at least n times. |
| {n,m}? | Matches the preceding pattern at least n times, but not more than m times. |
position
可选。搜索在字符串中的开始位置。如果省略，默认为1，这是第一个位置的字符串。
occurrence
可选。它是模式字符串中的第n个匹配位置。如果省略，默认为1。
match_parameter
可选。它允许你修改regexp_substr功能匹配的行为。它可以是以下的组合：
| Value | Description |
| 'c' | 区分大小写的匹配. |
| 'i' | 不区分大小写的匹配. |
| 'n' | Allows the period character (.) to match the newline character. By default, the period is a wildcard. |
| 'm' | expression is assumed to have multiple lines, where ^ is the start of a line and $ is the end of a line, regardless of the position of those characters in expression. By default, expression is assumed to be a single line. |
| 'x' | Whitespace characters are ignored. By default, whitespace characters are matched like any other character. |
Examples
找出匹配的数字
SELECT REGEXP_SUBSTR ('hello my phone is 520 ', '[0-9]+') FROM dual; --520
下面这个例子返回指定第三次出现的字符.
SELECT REGEXP_SUBSTR ('i like beijing tiananmen', '(\S*)(\s)', 1, 3)
FROM dual; --beijing
