Match string only if a word in the string does not exist

Question

I am trying to extract table names from a SQL script. I have the following string:

from db1.tableX tbx1 --quantity table
inner join tableY tbx2  on tbx1.xyz=tbx2.xyz

I don't want to match this string because tabley is not prefixed with a database name. The way I am trying to do this is by detecting the word "on" in the string before the database name.

My regex is matching tablex but also matches tbx1, which I dont want my expression to match.

I would like it to match only in this situation.

from db1.tableX tbx1 --quantity table
inner join db1.tableY tbx2 on tbx1.xyz = tbx2.xyz

My regex should give me tableX and tableY because they are prefixed by a database name.

Here is my expression:

(insert\s+into|from|inner\s+join|left\s+outer\s+join|join)\s+[\sa-zA-Z0-9_$#-]*\.\s*(?<table>[a-zA-Z0-9_]+)(?=\s+[a-zA-Z0-9_$#-]+)*

Answer 1

You can simplify your expression

(?<=\b(from|join)\s+[a-zA-Z0-9_$#-]*\.\s*)[a-zA-Z0-9_]+

It will yield the table name directly, without having to use named groups. I am using the pattern

(?<=prefix)find

This returns only the part "find" as match value. As prefix I use "from" or "join" followed by the database name the dot and possibly spaces.

\b is the beginning or end of a word.

The last part [a-zA-Z0-9_]+ is the table name.

It makes no difference whether you have an inner join, outer join etc., so I dropped this distinction.

Comments are ugly to detect, beacuse they can contain about anything and can appear about everywhere. Also you have two types of comments -- Until end of line and \* ... *\.

You could try to detect comments and spaces like this

(\s|--.*?$|\\\*.*?\*\\)+

Note that the backspace and the star have to be escaped, so \* becomes \\\* and *\ becomes \*\\.

$ denotes the end of line. The ? after .* ensures that not several comments are skipped at once, thus skipping meaningful text between them.

This will work in most cases; however, comment like structures could be included within string literals: SELECT 'hello -- world' AS greeting FROM .... This is not a comment! Without a full syntax analysis of the SQL text you will not be able to cover all possible cases. But you cannot do this with Regex. Regex has its limits.

Answer 2

Step by step:

1) http://regexr.com?33tto

I don't know why you've put that lookahead at the end of the match. I don't think you need it, so I removed it.

2) http://regexr.com?33ttr

Now we get to the problem that you've indicated. The problem is that in your database name matching [\sa-zA-Z0-9_$#-]* you've included whitespace as a valid char. I've removed that too.

3) http://regexr.com?33ttu

Is this the desired outcome?