REGEXPS

REGEXPS

The majority of matching in MUX is done with wildcard (“globbing”) patterns. There is a second type of matching, using regular expressions, that is available in certain circumstances.

For attributes that are $-commands or ^-listen-patterns, setting that attribute “regexp” (with @set <object>/<attribute>=regexp) causes patterns to be matched using regular expressions rather than globbing.

In a regular expression match, the substring of the string which matched the regexp pattern is %0; %1 through %9 are the substrings of the string which matched parenthesized expressions within the regexp pattern.

Regular expressions are extremely useful when you want to enforce a data type. For example, if you have a command where you want a player to enter a string and a number (+setnum <player>=<number>, for example), you might do it like this:

&DO_NUM Command Object=$\+setnum (.+)=([0-9]*): @va me=Data: %1 = %2
@set Command Object/DO_NUM = regexp

Then, +setnum cookies=30 would set VA to Data: cookies = 30. This eliminates your having to check to see if the player entered a number, since the regular expression matches only numbers. Furthermore, the + guarantees that there needs to be at least one character there, so a player can’t enter +setnum cookies= or +setnum =10 or similarly malformed input.

Also note that regexps are case-sensitive, but wildcard globbing is not. Some regular expression functions offer case-insensitive versions, some regexp can be made case-insensitive by putting (?i) at the start. Setting the CASE attribute flag is another way of selecting case-insensitivity.

Regular Expression Syntax

The following explanation is taken from Henry Spencer’s regexp(3) package, the regular expression library used in TinyMUX up to version 2.2. After that, the PCRE package is used, which has many more features. See a book that covers Perl’s regular expressions for details.

A regular expression is zero or more branches, separated by |. It matches anything that matches one of the branches.

A branch is zero or more pieces, concatenated. It matches a match for the first, followed by a match for the second, etc.

A piece is an atom possibly followed by *, +, or ?. An atom followed by * matches a sequence of 0 or more matches of the atom. An atom followed by + matches a sequence of 1 or more matches of the atom. An atom followed by ? matches a match of the atom, or the null string.

An atom is a regular expression in parentheses (matching a match for the regular expression), a range (see below), . (matching any single character), ^ (matching the null string at the beginning of the input string), $ (matching the null string at the end of the input string), a \ followed by a single character (matching that character), or a single character with no other significance (matching that character).

A range is a sequence of characters enclosed in []. It normally matches any single character from the sequence. If the sequence begins with ^, it matches any single character not from the rest of the sequence. If two characters in the sequence are separated by -, this is shorthand for the full list of ASCII characters between them (e.g. [0-9] matches any decimal digit). To include a literal ] in the sequence, make it the first character (following a possible ^). To include a literal -, make it the first or last character.

Regular Expression Examples

• The regexp pattern . is equivalent to the wildcard ?; it matches one and only one of an arbitrary character.
• The regexp pattern .+ is equivalent to the wildcard *; it matches one or more arbitrary characters. To match zero or more arbitrary characters, the regexp pattern is .*.
• To match a string of numbers, use: [0-9]+
• To match a string of letters only, use: [A-Za-z]+

Regular Expression Ambiguity

If a regular expression could match two different parts of the input string, it will match the one which begins earliest. If both begin in the same place but match different lengths, or match the same length in different ways, life gets messier, as follows.

In general, the possibilities in a list of branches are considered in left-to-right order, the possibilities for *, +, and ? are considered longest-first, nested constructs are considered from the outermost in, and concatenated constructs are considered leftmost-first. The match that will be chosen is the one that uses the earliest possibility in the first choice that has to be made. If there is more than one choice, the next will be made in the same manner (earliest possibility) subject to the decision on the first choice. And so forth.

For example, (ab|a)b*c could match abc in one of two ways. The first choice is between ab and a; since ab is earlier, and does lead to a successful overall match, it is chosen. Since the b is already spoken for, the b* must match its last possibility–the empty string–since it must respect the earlier choice.

In the particular case where no |s are present and there is only one *, +, or ?, the net effect is that the longest possible match will be chosen. So ab*, presented with xabbbby, will match abbbb. Note that if ab* is tried against xabyabbbz, it will match ab just after x, due to the begins-earliest rule. (In effect, the decision on where to start the match is the first choice to be made, hence subsequent choices must respect it even if this leads them to less-preferred alternatives.)