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If the square brackets notation does not give you enough precision or flexibility, you can use multicharacter collation elements, character classes, and equivalence classes, as follows: Multicharacter collation elements are relevant for certain languages. Valid values are predefined and depend on the NLS_SORT setting. Use [. and .] to enclose collation elements. Character classes give you more flexibility than the dash symbol between square brackets; for example, you can refer to alphabetic characters, numeric digits, alphanumeric characters, blank spaces, punctuation, and so on. Use [: and :] to enclose character classes. Equivalence classes allow you to match all accented and unaccented versions of a letter. Use [= and =] to enclose equivalence classes.

Before we look at some examples of how these regular expression operators work with the regular expression functions (in Listings 5-14 through 5-16), we need to discuss the syntax of the functions.

The four regular expression functions have the following syntax. You can specify regular expressions in their pattern argument. REGEXP_LIKE(text, pattern[, options]) REGEXP_INSTR(text, pattern[, pos[, occurrence[, return[, options]]]]) REGEXP_SUBSTR(text, pattern[, pos[, occurrence[, options]]]) REGEXP_REPLACE(text, pattern[, replace [, pos[, occurrence[, options]]]])

For all four functions, the first two arguments (text and pattern) are mandatory. These arguments provide the source text and the regular expression to search for, respectively. All of the remaining arguments are optional. However, function arguments can only be omitted from the right to the left. For example, if you want to specify a value for the options argument of the REGEXP_INSTR function, all six arguments are mandatory and must be specified. In REGEXP_INSTR, REGEXP_SUBSTR, and REGEXP_REPLACE, you can use the pos argument to specify from which position in text you want the search to start (the default value is 1), and with occurrence, you can specify how often you want to find the search pattern (the default value is 1). The options argument of all four of the functions and the return argument of the REGEXP_INSTR function require a bit more explanation.

You can influence the matching behavior of the regular expression functions with their options argument. Table 5-5 shows the values you can specify in the options function argument.

Case-insensitive search (no distinction between uppercase and lowercase) Case-sensitive search Allows the period (.) to match the newline character Treat text as multiple lines; ^ and $ refer to the beginning and end of any of those lines

You can specify one or more of these values. If you specify conflicting combinations, such as 'ic', the Oracle DBMS uses the last value (c) and ignores the first one.

The return option of the REGEXP_INSTR function allows you to influence the return value. By default, the position where the pattern was found is returned, but sometimes you want to know the position immediately after the found pattern. Of course, you can add the length of the pattern to the result of the function; however, using the return option is easier in that case. Table 5-6 shows the values you can specify in the return function argument. Table 5-6. Regular Expression Return Values

Position of the first character of the pattern found (default) Position of the first character after the pattern found

Let s look at an example of the REGEXP_LIKE function, using a SQL*Plus trick that will be explained in a later chapter. The ampersand character (&) in the WHERE clause of the query in Listing 5-14 makes SQL*Plus prompt for a value for text; therefore, you can repeat this query in the SQL buffer with the / command as often as you like, specifying different source text values to explore the effect of the search pattern.