A string pattern is a string composed of combinations of characters that can be used to describe all strings of a certain form. String patterns are used with some functions provided by Lua, for example to tell them to look in a string for substrings of a certain form. The following is an example of what can be done with string patterns:
As said before, string patterns are strings that look a little different and are used for a different purpose than what strings are usually used for. Here we will look at the basics of just what make a string pattern up. Here we will look at just what the different parts of a string pattern mean.
Guess what? You already know some string patterns! Any string is a pattern!
There's only so far we can go by using this kind of pattern matching. Sometimes, we want to match any of a set of characters. Here's an example:
The following table shows the meaning of each character class:
|%a||An uppercase or lowercase letter||aBcDeFgHiJkLmNoPqRsTuVwXyZ|
|%l||A lowercase letter||abcdefghijklmnopqrstuvwxyz|
|%u||An uppercase letter||ABCDEFGHIJKLMNOPQRSTUVWXYZ|
|%p||Any punctuation character||#^;,.|
|%w||An alphanumeric character - either a letter or a digit||aBcDeFgHiJkLmNoPqRsTuVwXyZ0123456789|
|%s||A whitespace character||, \n, and \r|
|%c||A control character|
|%x||A hexadecimal (Base 16) digit||0123456789ABCDEF|
|%z||The NULL character, '\0'|
|%f||The frontier pattern (not officially documented)|
|%bxy||The balanced capture. It matches x, y, and everything in between. It allows for the nesting of balanced captures as well. (Note: x and y must be different)||%b() captures everything between parentheses (including them).|
Any non-magic character (not one of
^$()%.*+-?), represents itself in a pattern. To search for a literal magic character, precede it by a
% - for example, to look for a percent symbol, use
Here's an example that shows how the
. character can be used.
. matches any character, and
%. matches an actual period.
One of the things you might notice about the character classes above is that they are all lowercase. Making them uppercase reverses their effect. For instance,
%s represents whitespace, but
%S represents any non-whitespace character.
%l represents a lowercase letter while
%L represents its compliment - any characters but a lowercase letter. Let's look at this example, which matches a digit, followed by five non-digits:
Character classes allow you to match any character. Quantifiers allow you to match any number of characters.
|?||Match 0 or 1 of the preceding character specifier|
|*||Match 0 or more of the preceding character specifier|
|+||Match 1 or more of the preceding character specifier|
|-||Match as few of the preceding character specifier as possible|
Let's say you have a string that contains a number, such as
"It costs 100 tix",
and you want to extract the number. If you know how many digits the number has, you could use the pattern
%d%d%d which would match three digits in a row. But what happens if you don't know how many digits there are? For this, you can use quantifiers. In this example, the
+ quantifier is suitable.
Now how does this work exactly? As we know, a character class followed by a '+' matches one or more repetitions. For this example, it means that it would match the first digits it finds until it reaches the end of the string or a non-digit.
The difference between
* is that
+ matches 1 or more characters, while
* matches 0 or more. This means that if the character class that is followed by this quantifier isn't represented in the string, it doesn't matter, because the match isn't required.
As you can see, it matches a digit, followed by a comma, any amount of whitespace (if there is any), and then another digit. If you had used
+, the second example would have returned nil, because
+ requires at least one match. The
* pattern is very useful when you have something in the string that is optional.
- quantifier is a little different from the previous two. Like
*, it matches 0 or more characters. However,
* try to match the longest possible sequence, whereas
- attempts to match the shortest possible sequence. Here's an example that shows the difference:
From the example, you see that the
- found the shortest possible sequence and stopped only at the second
/, while the
* matched the longest sequence and stopped at the last
/ in the string.
This concept is usually referred to as "greediness". Quantifiers that match the longest possible sequence are considered greedy, while ones that match the shortest possible sequence are considered non-greedy.
? quantifier is used to make certain characters in the string optional.
From the example you can see that the
? made the s and k optional, allowing the pattern to match "wii" and "wikis". However, only one k was allowed, so wikki was not matched
Anchors are characters in the pattern which ensure that the pattern matches at either the beginning and/or the end of the string.
Consider the following example, which matches any sequence of digits in a string. This example will be modified later on:
If you put a
^ character at the beginning of the pattern, then the pattern will match only at the beginning of the string. In other words, the match must contain the beginning of the string.
Let's try anchoring the pattern to the beginning of the string:
As you can see, it matched only the first sequence, because it was at the beginning of the string.
Also, consider the following example:
Even though there are sequences of digits, none of them were matched, because none of them were at the beginning of the string.
Additionally, if you put a
$ character at the end of the pattern, then the pattern will match only at the end of the string.
As you can see, only the last sequence was matched, because it was located at the end of the string.
Both anchors can be used in a pattern at the same time. Let's try it:
That didn't work very well. That's because there are non-digit characters between the beginning and the end of the string. Let's eliminate those characters:
That's more like it. As you can see, using both anchors can be useful for validating the entire string, instead of just parts of it.
Sets are used when a single character class cannot do the whole job. For instance, you might want to match both lowercase letters (%l) as well as punctuation characters (%p) using a single class. So how would we do this? Let's take a look at this example:
As you can see from the example, sets are defined by the
] around them. You also see that the classes for lowercase letters and punctuation are contained within. This means that the set will act as a class that represents both lowercase and punctuation, unlike if you used
%l%p, which would match a lowercase letter and a punctuation character following it.
You aren't restricted to using only character classes, though! You can also use normal characters to add to the set.
You can also specify a range of characters. A range is written as
x is the character at the start of the range, and
y is the character at the end of the range (ex: "0-9" for all digits). Let's see how this works in the following example:
As you can see, sequences of the characters "a" through "k", "q" through "z", and "3" through "6" were successfully matched.
There's still one last thing you can do. Like with character classes, sets have complements of themselves. These are indicated by using a
This pattern is the complement of
[%s,]. From the example, you can see that the complement of a set is defined by using the
^ character at the beginning of the set. All this does is reverse the meaning of the set.
Captures are used to get pieces of a string that match a capture. Captures are defined by parentheses around them. For instance, (%a%s) is a capture for a letter and a space character. When a capture is matched, it is then stored for future use. Let's look at this example:
Now what happens if you want to get a list by using captures? You can use string.gmatch to do this.
Note that 'key' and 'val' are actually referring to capture 1 and capture 2. The name does not matter, but it is still a good practice to choose a relevant name. As you can see, string.gmatch iterated through all the matches in the string and returned only the captures which is basically what captures are for, to capture a certain part of the string to use.
A final thing you can do with captures is that you can leave the captures empty. In these cases they will capture the current position on the string. This means that unlike the other, non-empty captures, a number is returned instead of a string. Look at this example:
From the example, once a match was found, string.match returned the first and second captures' positions in the string instead of returning the characters 'H' and '!'.