Lua's pattern matching rules
Pattern matching in httpd(8) is based on the implementation of the Lua scripting language and provides a simple and fast alternative to the regular expressions (REs) that are described in re_format(7). Patterns are described by regular strings, which are interpreted as patterns by the pattern-matching “find” and “match” functions. This document describes the syntax and the meaning (that is, what they match) of these strings.
A character class is used to represent a set of characters. The following combinations are allowed in describing a character class:
- (where x is not one of the magic characters ‘^$()%.*+-?’) represents the character x itself.
- (a dot) represents all characters.
- represents all letters.
- represents all control characters.
- represents all digits.
- represents all printable characters except space.
- represents all lowercase letters.
- represents all punctuation characters.
- represents all space characters.
- represents all uppercase letters.
- represents all alphanumeric characters.
- represents all hexadecimal digits.
- (where x is any non-alphanumeric character) represents the character x. This is the standard way to escape the magic characters. Any non-alphanumeric character (including all punctuation characters, even the non-magical) can be preceded by a ‘%’ when used to represent itself in a pattern.
- represents the class which is the union of all characters in
set. A range of characters can be specified by
separating the end characters of the range, in ascending order, with a
‘-’. All classes ‘%x’
described above can also be used as components in
set. All other characters in
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 ‘-’
The interaction between ranges and classes is not defined. Therefore, patterns like ‘[%a-z]’ or ‘[a-%%]’ have no meaning.
- represents the complement of set, where set is interpreted as above.
For all classes represented by single letters ( ‘%a’, ‘%c’, etc.), the corresponding uppercase letter represents the complement of the class. For instance, ‘%S’ represents all non-space characters.
The definitions of letter, space, and other character groups depend on the current locale. In particular, the class ‘[a-z]’ may not be equivalent to ‘%l’.
A pattern item can be
- a single character class, which matches any single character in the class;
- a single character class followed by ‘*’, which matches zero 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 one 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 zero 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 zero or one occurrence of a character in the class. It always matches one occurrence if possible;
- ‘%n’, for n between 1 and 9; such item matches a substring equal to the n-th captured string (see 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.
- ‘%f[set]’, a frontier pattern; such item matches an empty string at any position such that the next character belongs to set and the previous character does not belong to set. The set set is interpreted as previously described. The beginning and the end of the subject are handled as if they were the character ‘\0’.
A pattern is a sequence of pattern items. A caret ‘^’ at the beginning of a pattern anchors the match at the beginning of the subject string. A ‘$’ at the end of a pattern anchors the match at the end of the subject string. At other positions, ‘^’ and ‘$’ have no special meaning and represent themselves.
A pattern can contain sub-patterns enclosed in parentheses; they describe captures. When a match succeeds, the substrings of the subject string 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.
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: 2 and 4.
fnmatch(3), re_format(7), httpd(8)
Roberto Ierusalimschy, Luiz Henrique de Figueiredo, and Waldemar Celes, Patterns, Lua 5.3 Reference Manual, https://www.lua.org/manual/5.3/manual.html#6.4.1, Lua.org, PUC-Rio, June 2015.
The first implementation of the pattern rules were introduced with Lua 2.5. Almost twenty years later, an implementation based on Lua 5.3.1 appeared in OpenBSD 5.8.
The pattern matching is derived from the original implementation of the Lua scripting language written by Roberto Ierusalimschy, Waldemar Celes, and Luiz Henrique de Figueiredo at PUC-Rio. It was turned into a native C API for httpd(8) by Reyk Floeter <firstname.lastname@example.org>.
A notable difference with the Lua implementation is the position in the string returned by captures. It follows the C-style indexing (position starting from 0) instead of Lua-style indexing (position starting from 1).