Pattern Matching

Introduction

One of the features included in dos.library is a series of functions to do standard pattern matching. Using a set of standard string matching tokens, any application can use these functions to test if a particular string matches a pattern. The Amiga OS uses these functions for processing file name strings for its new directory scanning functions.

These functions can be used in every circumstance where you would like to enable the user to enter a pattern to indicate more than one target string. Using these functions not only makes it unnecessary to implement your own pattern matching routines, but by using the familiar DOS pattern tokens in your application, it is easier for the user to learn how to use your application.

Patterns

An AmigaDOS pattern matching string is a combination of alphanumeric characters and a series of special token characters. These token characters are part of the ASCII character set and they denote such things as string matching wildcards, string repetitions, and string negation. Pattern matching strings can use parentheses to delimit pattern matching substrings.

 ?

The question mark matches any single character. For example, the pattern matching string "A?B" matches any string that is three letters long, that starts with an "A" and ends with a "B".

#

The number sign matches strings containing zero or more repetitions of the expression that immediately follows the # in the pattern matching string. For example, the pattern matching string "#A" matches any string that consists of one or more of the "A" character. The pattern matching string "#?" matches any non-NULL string. The # can apply to entire substrings delimited by parentheses. For example, the pattern string "#(AB)" matches any string consisting of one or more repetitions of the substring "AB" (AB, ABAB, ABABAB...).

 %

Matches the NULL string.

|

This is the OR symbol. This matches strings that contain the expressions on either side of the OR sign. The expressions and the OR symbol need to be enclosed in parentheses. For example, the pattern matching string "(A|B)" matches the string "A" or the string "B". The pattern matching string A(B|%|C) matches the strings "AB", "A", and "AC".

~

The tilde negates the expression that follows it. All strings that do not match the expression that follows the tilde will match the expression with the tilde. For example, the pattern matching string "~(#?.info)" matches any string that does not match the string "#?.info" (does not end with the substring ".info").

*

The star is provided as an synonym to "#?". This is an option which can be turned on. Note that the star can not be used by itself on all non-FileSystem devices, like a logical device name assigned to a directory on a file system. For example:

Assign A: dh0:tmp
cd a:
list *

will produce an error. DOS preferences can be used to enable or disable this feature.