Using patterns in REPLACE

I'm not entirely clear on what you're trying to acheieve (e.g. what are you wanting to replace "field_1=300" with, and is it the exact string "field_1=300" that you're looking for, or just the field name, i.e. "field_1"?).

Also, could you paste in the code you've written so far?

Here's a simple script which will extract the current value of a given field name:

DECLARE @str VARCHAR(100),
        @str_tmp VARCHAR(100),
        @field_pattern VARCHAR(10),
        @field_val INT;

SET @str = 'fieldA=23 OR field_1=300 OR fieldB=4';
SET @field_pattern = 'field_1='

-- This part will extract the current value assigned to the "@field_pattern" field
IF CHARINDEX(@field_pattern, @str) > 0
BEGIN
    SELECT @str_tmp = SUBSTRING(@str, 
                        CHARINDEX(@field_pattern, @str) + LEN(@field_pattern),
                        LEN(@str)
                        );

    SELECT @field_val = CAST(SUBSTRING(@str_tmp, 1, CHARINDEX(' ', @str_tmp)-1) AS INT);
END

PRINT @field_val

If you want to replace the value itself (e.g. replacing "300" in this case with "600"), then you could add something like this:

DECLARE @new_val INT;
SET @new_val = 600;
SET @str = REPLACE(@str, (@field_pattern+CAST(@field_val AS VARCHAR)), (@field_pattern+CAST(@new_val AS VARCHAR)));
PRINT @str;

Which would give you "fieldA=23 OR field_1=600 OR fieldB=4".

Cheers, Dave