regexp_instr tips

Question: What does the regexp_instr operator do?  Can you show an example of using regexp_instr?

Answer:  The regexp_instr operator shows matching patterns within a string.

The regexp_instr function is a very powerful to extract patterns from inside a string.  Let's take a closer look at the prototype for the regexp_instr function:

regexp_instr (
   string,
   pattern,
   position,
   occurrence,
   return-option,
   parameters)

The last argument is the most important.  The "parameters" argument can be a combination of these formats:

i - Used to introduce case-insensitively
c - Keeps case (default value)
n - to make the dot (.) match new lines as well
m - to make ^ and $ match beginning and end of a line in a multi-line string

The "i" parameter is used to force case-insensitive parsing.  Just like the common UNIX "grep –i" command, we can use the lowercase "i" to extract both "Acre" and "acres".  The lowercase 'i" is also the last argument to the regexp_like function.

Oracle 10g introduced regular expression functions in SQL with the functions REGEXP_SUBSTR, REGEXP_REPLACE, REGEXP_INSTR and REGEXP_LIKE.

SELECT
   ENAME,
   REGEXP_SUBSTR(ENAME,'DAM') SUBSTR,
   REGEXP_INSTR(ENAME, 'T') INSTR,
   REGEXP_REPLACE(ENAME,'AM','@') REPLACE,
   REGEXP_COUNT(ENAME, 'A') COUNT
FROM
   EMP
WHERE
   REGEXP_LIKE(ENAME,'S');

ENAME      SUBSTR          INSTR REPLACE         COUNT
---------- ---------- ---------- ---------- ----------
SMITH                          4 SMITH               0
JONES                          0 JONES               0
SCOTT                          4 SCOTT               0
ADAMS      DAM                 0 AD@S                2
JAMES                          0 J@ES                1

REGEXP_SUBSTR returns the substring DAM if found, REGEXP_INSTR returns the position of the first 'T', REGEXP_REPLACE replaces the strings 'AM' with  '@' and REGEXP_COUNT counts the occurrences of 'A'. REGEXP_LIKE returns the strings that contain the pattern 'S'.

SELECT
   REGEXP_SUBSTR('Programming','[[:alpha:]]+',1,2)
FROM
   DUAL;

REGEXP
------
Oracle

 

This function is an advanced extension of the already existing INSTR function, which returns the location of the expression pattern in the source string.

 

The prototype of the REGEXP_INSTR function is shown below,

 

 

·         Source_string: The string to be searched for.

 

·         Search_pattern: The regular expression pattern that is to be searched for in the source string. This can be a combination of the POSIX and the Perl-influenced metacharacters mentioned in the above section.

 

·         Start_position: This is an optional parameter. This determines the position in the source string where the search starts. By default, it is 1, which is the starting position of the source string.

 

·         Match_occurrence: This is an optional parameter. This determines the appearance of the search pattern. By default, it is 1, which is the first appearance of the search pattern in the string.

 

·         Return_option: This is an optional parameter. By default, it is 0.

 

è If 0, the position of the first occurrence is returned.

 

è If 1, the position of the character after the first occurrence is returned.

 

·         Match_modifiers: This is an optional parameter. This parameter allows us to modify, the matching behavior of the function. The valid range of options is mentioned in the Pattern Matching Modifiers section explained above.

 

Text Position Search

The basic operation, which can be done using this function is to find the position of a character or a series of characters in the source string as shown below. The below statement returns the position of the first occurrence of the string INSTR in the source string with the return option as 0.

 

 

Result:

8

 

When the return option is changed to 1, the function returns the position of the character after INSTR, that is, the position of the space is returned in the below example,

 

 

Result:

13

Digit Position Search

The below statement, searches for the position of numeric digits present in the source string using the Perl influenced metacharacter \d. This function returns the position of the first digit in the source string, i.e., the position of the digit 1 is returned in the below example.

 

 

Result:

50

Vowels Position Search

The below listing searches and returns the position of the first vowel in the source string with case sensitivity check turned OFF manually using the match modifier set to i.

 

 

Result:

2

DOT Count

The position of the second occurring DOT character in the source string can be found using the below query statement. The search pattern “\.” searches for the literal DOT in the source string. As DOT is a metacharacter, it is escaped using the escape operator backslash.

 

 

Result:

16

 

 

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