regexp语句中没有前面的字符

因此，我尝试在regexp语句中使用否定回溯，并在网上查看了其他解决方案，但它们似乎对我不起作用，所以很明显我做错了什么-

我正在寻找第一行的回报，但其他的应该是空的。基本上我需要CT胸部或肺部 有什么帮助吗

with test (id, description) as (
  select 1, 'CT CHEST HIGH RESOLUTION, NO CONTRAST'                         from dual union all --want this
  select 2, 'INJECTION, THORACIC TRANSFORAMEN EPIDURAL, NON NEUROLYTIC W IMAGE GUIDANCE.'                 from dual union all   --do not want this
  select 3, 'The cow came back. But the dog went for a walk'    from dual) --do not want this 
  select id, description, regexp_substr(description, '(?<![a-z]ct).{1,20}(CHEST|THOR|LUNG)',1,1,'i') from test;

works

利用Oracle子EXPRESSION参数检查CT

我将利用子表达式的使用来使用如下模式：

'regexp_substr(description, '(^| )((ct ).*((CHEST)|(THOR)|(LUNG)))', 1, 1,'i', 2)`

-子表达式1查找行首或空格：^|

-用于查找“CT”的子表达式3：CT

-允许其他字符：*

-下颌骨5,6,7：胸部|托尔|肺

-包含子表达式3和子表达式4的子表达式2

我使用最后一个可选参数来标识我想要的子表达式2

    WITH test (id, description) as (
          SELECT 1
               , 'CT CHEST HIGH RESOLUTION , NO CONTRAST'
            FROM dual
           UNION ALL --want this
          SELECT 2
               , 'INJECTION , THORACIC TRANSFORAMEN EPIDURAL , NON NEUROLYTIC W IMAGE GUIDANCE.'
            FROM dual
           UNION ALL --do not want this
          SELECT 3
               , 'The cow came back. But the dog went FOR a walk'
    FROM dual
         ) --do not want this
  SELECT id
       , description
       , regexp_substr(description, '(^| )((ct ).*((CHEST)|(THOR)|(LUNG)))', 1, 1,'i', 2)
    FROM test;

---

仅供参考，oracle数据库不支持“向后看”或“向前看”。啊，那可不好玩！同意，“向前看”和“向后看”是正则表达式工具箱中很好的工具，非常感谢。oralce对正则表达式的实现还不错，但是你必须克服缺乏前瞻性和滞后性的缺点。使用子表达式是人们用来弥补这一缺陷的一种方法。

    WITH test (id, description) as (
          SELECT 1
               , 'CT CHEST HIGH RESOLUTION , NO CONTRAST'
            FROM dual
           UNION ALL --want this
          SELECT 2
               , 'INJECTION , THORACIC TRANSFORAMEN EPIDURAL , NON NEUROLYTIC W IMAGE GUIDANCE.'
            FROM dual
           UNION ALL --do not want this
          SELECT 3
               , 'The cow came back. But the dog went FOR a walk'
    FROM dual
         ) --do not want this
  SELECT id
       , description
       , regexp_substr(description, '(^| )((ct ).*((CHEST)|(THOR)|(LUNG)))', 1, 1,'i', 2)
    FROM test;