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Patterns
Note: This tutorial requires some knowledge of string manipulation.
Patterns are very useful tools for string manipulation. They allow you to do operations on all numbers in a string,
Classes
Character Class:
A character class is used to represent a set of characters. The following are character classes and their representations:
- x — Where x is any non-magic character (^$()%.[]*+-?), x represents itself
- . — Represents all characters (#32kas321fslk#?@34)
- %a — Represents all letters (aBcDeFgHiJkLmNoPqRsTuVwXyZ)
- %bxy — See Pattern Items below
- %c — Represents all control characters (all ascii characters below 32 and ascii character 127)
- %d — Represents all base-10 digits (0-9)
- %l — Represents all lower-case letters (abcdefghijklmnopqrstuvwxyz)
- %p — Represents all punctuation characters (#^;,.) etc.
- %s — Represents all space characters
- %u — Represents all upper-case letters (ABCDEFGHIJKLMNOPQRSTUVWXYZ)
- %w — Represents all alpha-numeric characters (aBcDeFgHiJkLmNoPqRsTuVwXyZ0123456789)
- %x — Represents all hexadecimal digits (0123456789ABCDEF)
- %z — Represents the ascii character with representation 0 (the null terminator)
- %x — Represents (where x is any non-alphanumeric character) the character x. This is the standard way to escape the magic characters. Any punctuation character (even the non magic) can be preceded by a '%' when used to represent itself in a pattern. So, a percent sign in a string is "%%"
Here are some examples:
An upper-case version of any of these classes results in the complement of that class. For instance, %A will represent all non-letter characters. Cool thing? You can combine character classes! Here's another example:
Modifiers
In Lua, modifiers come after character classes and are used for repetitions and optional parts, which is why they're useful:
- + — 1 or more repetitions
- * — 0 or more repetitions
- - — (minus sign) also 0 or more repetitions
- ? — optional (0 or 1 occurrence)
I'll start with the simplest one: the ?. This makes the character class optional, and if it's there, captures 1 of it. That sounds complex, but is actually really simple, so here's an example:
The O is the O in Once.
Nothing is returned the second time because the digit didn't need to be there.
The + symbol requires at least one instance of that class, and will get the longest string of that class. Here's an example:
Once is the first set of only letters—the space between once and upon is not a letter, so the pattern stops there.
The second time nil was returned because the pattern required the digit to be there, but it wasn't.
The * symbol is like a combination of the + and ? modifiers. It matches the longest sequence of the character class, but it doesn't have to be there. Here's an example of it matching a floating-point (decimal) number, without requiring the decimal:
The first result was the whole number, but there wasn't a decimal point nor were there numbers after the decimal point
The second result was the floating-point number, because it had a decimal and numbers after it
The - symbol is like the * symbol; actually, there's only one difference: It matches the shortest sequence of the character class. Here's an example showing the difference:
The first result was everything from the first opening parenthesis to the last closing parenthesis
The second result everything from the first opening parenthesis to the first closing parenthesis
Sets
- [set] represents the class which is the union of all characters in the set. You define a set with brackets, like [%a%d]. A range of characters may be specified by separating the end characters of the range with a '-'. All classes described above may also be used as components in set. All other characters in a set represent themselves. For example, [%w_] (or [_%w]) represents all alphanumeric characters plus the underscore, [0-7] represents the octal digits, and [0-7%l%-] represents the octal digits plus the lowercase letters plus the '-' character.
The interaction between ranges and classes is not defined. Therefore, patterns like [%a-z] or [a-%%] have no meaning.
- [^set] represents the complement of set. The complement of a set is everything not in that set. [^set] is to [set] as %A is to %a.
The first result is the first vowel
The second result is the first non-vowel character (a space, which was right after the I)
The 0 was the first octal digit, late in the string.
Pattern Items
Alright, now it's time to explain what a pattern item is. A pattern item may be:
- a single character class, which matches any single character in the class;
- a single character class followed by '*', which matches 0 or more repetitions of characters in the class. These repetition items will always match the longest possible sequence;
- a single character class followed by '+', which matches 1 or more repetitions of characters in the class. These repetition items will always match the longest possible sequence;
- a single character class followed by '-', which also matches 0 or more repetitions of characters in the class. Unlike '*', these repetition items will always match the shortest possible sequence;
- a single character class followed by '?', which matches 0 or 1 occurrence of a character in the class;
- %n, for n between 1 and 9; such item matches a substring equal to the n-th captured string (see Captures below);
- %bxy, where x and y are two distinct characters; such item matches strings that start with x, end with y, and where the x and y are balanced. This means that, if one reads the string from left to right, counting +1 for an x and -1 for a y, the ending y is the first y where the count reaches 0. For instance, the item %b() matches expressions with balanced parentheses.
The pattern found two opening parentheses, so it went to find two closing ones, trapping everything in between the first opening parenthesis and the second closing parenthesis.
Patterns
A pattern is a sequence of pattern items. A '^' at the beginning of a pattern anchors the match at the beginning of the string. A '$' at the end of a pattern anchors the match at the end of the string. At other positions, '^' and '$' have no special meaning and represent themselves. Also, a pattern cannot contain embedded zeroes. Use %z instead. Here's an example of a pattern:
The first result was the entire string because the entire string contained only alpha-numeric characters, punctuation marks, and spaces!
The second result matched only the letters at the start of the string.
Captures
A pattern may contain sub-patterns enclosed in parentheses; they describe captures. When a match of a capture succeeds, the substring that match captures are stored (captured) for future use. Captures are numbered according to their left parentheses. For instance, in the pattern "(a*(.)%w(%s*))", the part of the string matching "a*(.)%w(%s*)" is stored as the first capture (and therefore has number 1); the character matching "." is captured with number 2, and the part matching "%s*" has number 3. Whaaaaat??? Here:
The first result is because the first digit is at number:sub(1,1) and the second digit is at number:sub(2,2).
The second string had the parentheses represent a capture of one digit immediately followed by another. So, what a capture does is return whatever the function returns, which, for string.find, are the locations, as well as the matched substring. What's inside the parentheses is the substring that is being matched. So, the %d%d was the substring that was to be matched, and the return values are the 1 and the 2, which are the values the function returns, followed by the matched substring, which is 55.
As a special case, the empty capture () captures the current string position (a number). For instance, if we apply the pattern "()aa()" on the string "flaaap", there will be two captures: 3 and 5.
Quiz
Quiz on Patterns
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Note: This is a quiz. The answers are not explained. If you don't understand, review this page, then ask in the forums {{{2}}}
