In the previous section we looked at format strings, which provide a convenient way to create consistent, well-formatted strings that can contain unique values. In this chapter we look at patterns, which are roughly the opposite: patterns allow us to extract values from well-formatted strings.
A pattern defines a specification that describes the content of a strings. When a pattern is compared to a "target" string, if the pattern describes the string we say that the target "matches the pattern". Patterns are generally composed of a sequence of smaller patterns, called "atoms". These atoms are arranged in a specific sequence to create the specification.
As a simple example, let's build a pattern that matches phone numbers, then we will describe the process in more detail in the coming sections. We will assume phone numbers matching the format:
+1(234)567-8910
The first step is to break this string into a sequence of components:
+[country code]([area code])[prefix]-[line number]
Next, we need to specify each component in terms of regular expressions, then combine them into the pattern. One possible final pattern might be:
+%d[(]%d%d%d[)]%d%d%d-%d%d%d%d
which is made up of:
- A literal
+ - The "character class"
%d, which specifies that the characters in those locations must be digits, - Literal
(and)surrounding a sequence of 3 digits%d - Sequences of 3 and 4 digits, separated by a literal
-
If we apply this pattern to our target phone number, the target string is returned (which is a truthy value indicating that a match occurred. For comparison, we include a few other target strings with slightly different format, which produce nil (i.e. falsy) values indicating that a match did not occur.
local pattern = "+%d[(]%d%d%d[)]%d%d%d[-]%d%d%d%d"
print(string.match("+1(234)567-8910", pattern)) -- +1(234)567-8910
print(string.match("+1(23)4567-8910", pattern)) -- nil
print(string.match("(234)567-8910", pattern)) -- nil
With that quick introduction, let's learn how to create patterns.