Wusage 7.1 Manual: Patterns and Regular Expressions


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Wusage 7.1 Manual

Patterns and Regular Expressions

Many of the configuration file options, such as Allowed Documents (allow), Items to be Charted (totalsgroup) and Rewrite Rules (rewrite), allow the use of patterns and regular expressions. Patterns and regular expressions are used to match several, many or all documents, sites or other items with a single, simple rule.

Patterns: A Simple Approach

The simplest pattern is a document name (or site name). For instance:

/directory/name.html

This pattern matches only the single document /directory/name.html. If it is listed for the Ignored Documents (ignore) option, then that single document only will be ignored in all statistics produced by wusage.

A slightly more complex pattern uses an * character to match any number of characters. This is identical to the way both MSDOS and Unix use the * character for commands such as Unix ls and MSDOS dir. For instance:

*.gif

This pattern matches all document names which end in the letters .gif. This is useful in the Ignored Documents (ignore) option, where it instructs Wusage to completely ignore all accesses to GIF-format images.

Note: the Remove These Suffixes (suffixes) option is applied first, before options such as Allowed Documents (allow) and Ignored Documents (ignore). If you want to write a pattern that matches a document such as /index.html, take into account that the index.html part will be removed by the standard suffixes option and use the / by itself. For the index file of a subdirectory, the slash will also be removed to combine all accesses to that index into one document name.

The * character can appear more than once, and it can appear at any point in the pattern. (This is identical to the way Unix and Windows 95/98/NT use the * symbol in commands such as dir.)

The ? character can also be used. ? matches any one character in the document name or site name.

The | character can be used to separate distinct patterns on the same line. If a document name or site name matches any of the patterns separated by the | character on that line, it is considered to be a match for the complete pattern.

If you wish to negate the entire pattern, matching only items that do not match the pattern, begin the pattern with a single ! character.

Regular Expressions: A Sophisticated Approach

A regular expression must begin with @. This character signifies that the rule is a regular expression, and not an ordinary pattern.

If you wish to negate the entire regular expression, matching only items that do not match the regular expression, follow the @ with a single ! character.

The remainder of the regular expression consists of zero or more branches, separated by `|'. The entire regular expression matches anything that matches one of the branches.

A branch consists of 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 zero or more matches of the atom. An atom followed by `+' matches a sequence of one or more matches of the atom. An atom fol- lowed 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 char- acter), 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 char- acters 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.

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 con- catenated constructs are considered leftmost-first. The match that will be chosen is the one that uses the earli- est 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*', pre- sented 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.)


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Glossary of Frequently Used Terms

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