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Oracle 10g New Features

 

Oracle 10g Regular Expression Support

 

 

Regular Expression Support

It is a method for simple and complex patterns for searching and manipulating a text. You can search, extract, format, and manipulate a text in the database. At the beginning, it appears that the syntax is not very intuitive but by second look it may look easy. The technique more reflects as UNIX style regular expressions.

 

Interfaces: Oracle Regular Expressions are implemented by the following functions available in SQL and PL/SQL.

  • REGEXP_LIKE,

  • REGEXP_REPLACE,

  • REGEXP_INSTR, and

  • REGEXP_SUBSTR

 

Metacharacters: The following is a list of supported Oracle metacharacters use in Oracle Regular Expressions.

 

Syntax

Description

Classification

.

Match any character

Dot

a?

Match ‘a’ zero or one time

Quantifier

a*

Match ‘a’ zero or more time

Quantifier

a+

Match ‘a’ one or more time

Quantifier

a|b

Match either ‘a’ or ‘b’

Alternation

a{m}

Match ‘a’ exactly m times

Quantifier

a{m,}

Match ‘a’ at least m times

Quantifier

a{m,n}

Match ‘a’ between m and n times

Quantifier

[abc]

Match either ‘a’ or ‘b’ or ‘c’

Bracket Expression

(…)

Group an expression

Subexpression

\n

Match nth subexpression

Backreference

[:cc:]

Match character class in bracket expression

Character Class

[.ce.]

Match collation element in bracket expression

Collation Element

[=ec=]

Match equivalence class in bracket expression

Equivalence Class

 

NOTE:

It is important to realize that the results of a particular regular expression query could be different under a different locale such as changing NLS_SORT, Case and Accent Sensitivity, etc.

 

Character Classes:  They are sensitive to the underlying character set such as the [:lower:] character class.

 

The following is a list of Oracle supports character classes, based on character class definitions in NLS classification data:

 

Character Class Syntax

Meaning

[:alnum:]

All alphanumeric characters

[:alpha:]

All alphabetic characters

[:blank:]

All blank space characters.

[:cntrl:]

All control characters (nonprinting)

[:digit:]

All numeric digits

[:graph:]

All [:punct:], [:upper:], [:lower:], and [:digit:] characters.

[:lower:]

All lowercase alphabetic characters

[:print:]

All printable characters

[:punct:]

All punctuation characters

[:space:]

All space characters (nonprinting)

[:upper:]

All uppercase alphabetic characters

[:xdigit:]

All valid hexadecimal characters

 

Example: Consider a simple query to convert the ‘McLean’ city name to a more readable format. You should look for any instance for a lower case letter immediately followed by an upper case letter. Your query should record these two letters in backreferences by using subexpressions, then replaces the first one, followed by a space, then followed by the second letter:

SQL> SELECT

                        REGEXP_REPLACE(‘McLean’,

                        ‘([[:lower:]])([[:upper:]])’, ‘\1 \2’) as “City”

            FROM dual;

 

Equuivalence Class: It allows searching for all characters that have a common base letter.

 

Example:

SQL> SELECT

            REGEXP_SUBSTR

            (‘iSelfSchooling NOT ISelfSchooling’, ‘[[=i=]]SelfSchooling’) as name

            FROM dual;

 

 

How to use REGULAR EXPRESSIONS in Oracle?

Keep this in your mind that these functions support CHAR, VARCHAR2, CLOB, NCHAR, NVARCHAR, and NCLOB datatypes.

 

REGEXP_LIKE function

It returns a Boolean indicating whether the pattern matched or not.

 

Example: Consider to write an expression that could search for common inflections of the verb ‘try’. The following regular expression will match try, trying, tried, and tries.

SQL> SELECT

                       REGEXP_LIKE (‘We are trying to make the subject easier.’,

                                                ‘tr(y(ing)? | (ied) | (ies))’) as REGEXT_SAMPLE

            FROM dual;

 

REGEXP_SUBSTR function

It returns the actual data that matches the specified pattern.

 

Example: Consider to write an expression that could return the ‘trying’ specified pattern.

SQL> SELECT

                       REGEXP_SUBSTR (‘We are trying to make the subject easier.’,

                                                ‘tr(y(ing)? | (ied) | (ies))’) as REGEXT_SAMPLE

            FROM dual;

 

REGEXP_INSTR function

It returns the character position of either the beginging or end of the match.

 

Example: Consider to write an expression that could return the position of ‘trying’ specified pattern.

SQL> SELECT

                       REGEXP_INSTR (‘We are trying to make the subject easier.’,

                                                ‘tr(y(ing)? | (ied) | (ies))’) as REGEXT_SAMPLE

            FROM dual;

 

REGEXP_REPLACE function

It looks for an occurrence of a regular expression and replaces it with the contents of a supplied text literal.

 

Example:

SQL> SELECT

                       REGEXP_REPLACE (‘We are trying to make the subject easier.’,

                                                ‘.’, ‘ for you.’ ) as REGEXT_SAMPLE

            FROM dual;

 

Or, query a list of all employees’ name that hired between 1996 and 1999.

SQL> SELECT ename FROM emp

            WHERE REGEXP_REPLACE

                        (TO_CHAR(hire_date, ‘YYYY’), ‘^199[6-9]$’);

You used ‘^’ to indicate that the beginning of the line has to be 199, and [-] with $ to specify range of valid characters.

 

Occurance in the REGEXP functions

All fuctions take an occurrence that specifies you require the nth matching expression in REGEXP_SUBSTR and REGEXP_INSTR, the default for which is 1.

 

Example: Consider extracting the 3th field being the Oracle system idenfication in a column.

SQL> SELECT

            REGEXP_SUBSTR(‘system/password@myhost:1521:mysid’,

            ‘[^:]+’, 1, 3) as “SID name”

            FROM dual;

 

You also can the REGEXP function in the DDL statement. Consider to configure a column to allow only alphabetical characters within a VARCHAR2 column.

SQL> CREATE TABLE mytest (c1 VARCHAR2(20),

                        CHECK (REGEXP_LIKE(c1, ‘^[[:alpha:]]+$)));

 

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