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c#regexregex-lookaroundsregex-group

Match specific string not preceded with a word and any amount of whitespaces using .NET regex

I am using

Regex: (?<!WHEN\s)EMP_ID

Query: SELECT EMP_ID, CASE WHEN EMP_ID > 115 THEN 'greater' WHEN EMP_ID < 115 THEN 'lower' END AS TEST

Matches: 1 match ( EMP_ID)

But if I add any spaces after WHEN in this query, then it will show 2 matches of 'EMP_ID', which is wrong. Or if I use the where condition in my query and use this column name then also it will give me 2 matches of the column name.

How to correct this regex or use a different method to solve this issue in C#?

Solution

  • You can use

    (?<!WHEN\s+)EMP_ID

    Or, with whole word matching:

    \b(?<!\bWHEN\s+)EMP_ID\b

    See the .NET regex demo.

    Since .NET regex flavor allows unknown length patterns inside lookbehinds, the (?<!WHEN\s+) works and fails any location that is immediately preceded with WHEN + one or more whitespaces.