REGEX XML question - Finding value of a name value pair - regex

Question

I have always try to do quick Regex query on XML tags. Yep, I have been told it is not a good idea an you should load it in an object but sometimes like this, it is a in Oracle db blob. I need to get the VALUE for a specific NAME in a name value pair XML like this :
<entry>
<string>NAME</string>
<string>VALUE</string>
</entry>
Is there a way to do this with REGEX

Answer 1

You should parse XML using an XML Parser and in Oracle you can use XMLQUERY:
SELECT XMLQUERY(
'/root/entry/string[1][text()="NAME2"]/../string[2]/text()'
PASSING XMLTYPE( xml, NLS_CHARSET_ID('UTF8') )
RETURNING CONTENT
) AS value
FROM table_name;
Or XMLTABLE:
SELECT value
FROM table_name
CROSS APPLY XMLTABLE(
'/root/entry'
PASSING XMLTYPE( xml, NLS_CHARSET_ID('UTF8') )
COLUMNS
name VARCHAR2(20) PATH './string[1]',
value VARCHAR2(20) PATH './string[2]'
)
WHERE name = 'NAME2';
Which for the sample data:
CREATE TABLE table_name ( xml BLOB );
DECLARE
value CLOB := '<root>
<entry>
<string>NAME1</string>
<string>VALUE1</string>
</entry>
<entry>
<string>NAME2</string>
<string>VALUE2</string>
</entry>
<entry>
<string>NAME3</string>
<string>VALUE3</string>
</entry>
<entry>
<string>NAME4</string>
<string>VALUE4</string>
</entry>
</root>';
dest_offset INTEGER := 1;
src_offset INTEGER := 1;
lang_context INTEGER := DBMS_LOB.DEFAULT_LANG_CTX;
result BLOB;
warning INTEGER;
warning_msg VARCHAR2(50);
BEGIN
DBMS_LOB.CreateTemporary(
lob_loc => result,
cache => TRUE
);
DBMS_LOB.CONVERTTOBLOB(
dest_lob => result,
src_clob => value,
amount => LENGTH( value ),
dest_offset => dest_offset,
src_offset => src_offset,
blob_csid => DBMS_LOB.DEFAULT_CSID,
lang_context => lang_context,
warning => warning
);
INSERT INTO table_name ( xml ) VALUES ( result );
END;
/
Both outputs:
| VALUE |
| :----- |
| VALUE2 |
Can you do it with a regular expression? Yes:
SELECT REGEXP_SUBSTR(
TO_CLOB( xml ),
'<entry>\s*<string>NAME2</string>\s*<string>([^<]*)</string>\s*</entry>',
1,
1,
'c',
1
) AS value
FROM table_name
Which outputs:
| VALUE |
| :----- |
| VALUE2 |
db<>fiddle here
However, you shouldn't as the XML parsing functions take an XPATH which specifies where it should look for the data. The regular expression will just treat the value as a string and look for the first match even if it is not in the expected place in the XML hierarchy.
For example, if your data is:
<root>
<entry>
<string>NAME1</string>
<string>VALUE1</string>
<other><entry><string>NAME2</string><string>NOT THIS</string></entry></other>
</entry>
<entry>
<string>NAME2</string>
<string>VALUE2</string>
</entry>
</root>
Then XMLQUERY and XMLTABLE will find the correct value but the regular expression outputs:
| VALUE |
| :------- |
| NOT THIS |
db<>fiddle here
Or, if your data suddenly has an attribute:
<root>
<entry>
<string>NAME1</string>
<string>VALUE1</string>
</entry>
<entry>
<string>NAME2</string>
<string attr="attr value">VALUE2</string>
</entry>
</root>
Then parsing with the regular expression will fail and return NULL.
db<>fiddle here
So, don't use a regular expression, use a proper XML parser.