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This text is copied from the Glimpse manual page. I have tried to remove things that do not apply to the lxr searcher, but beware, some things might have slipped through. I'll try to put together something better when I get the time. For more information on glimpse go to the Glimpse homepage.

Patterns

glimpse supports a large variety of patterns, including simple strings, strings with classes of characters, sets of strings, wild cards, and regular expressions (see Limitations).

Strings

Strings are any sequence of characters, including the special symbols `^' for beginning of line and `$' for end of line. The following special characters (`$', `^', `*', `[', `^', `|', `(', `)', `!', and `\' ) as well as the following meta characters special to glimpse (and agrep): `;', `,', `#', `>', `<', `-', and `.', should be preceded by `\\' if they are to be matched as regular characters. For example, \\^abc\\\\ corresponds to the string ^abc\\, whereas ^abc corresponds to the string abc at the beginning of a line.

Classes of characters

a list of characters inside [] (in order) corresponds to any character from the list. For example, [a-ho-z] is any character between a and h or between o and z. The symbol `^' inside [] complements the list. For example, [^i-n] denote any character in the character set except character 'i' to 'n'. The symbol `^' thus has two meanings, but this is consistent with egrep. The symbol `.' (don't care) stands for any symbol (except for the newline symbol).

Boolean operations

Glimpse supports an `AND' operation denoted by the symbol `;' an `OR' operation denoted by the symbol `,', a limited version of a 'NOT' operation (starting at version 4.0B1) denoted by the symbol `~', or any combination. For example, pizza;cheeseburger' will output all lines containing both patterns. '{political,computer};science' will match 'political science' or 'science of computers'.

Wild cards

The symbol '#' is used to denote a sequence of any number (including 0) of arbitrary characters (see Limitations). The symbol # is equivalent to .* in egrep. In fact, .* will work too, because it is a valid regular expression (see below), but unless this is part of an actual regular expression, # will work faster. (Currently glimpse is experiencing some problems with #.)

Combination of exact and approximate matching

Any pattern inside angle brackets <> must match the text exactly even if the match is with errors. For example, <mathemat>ics matches mathematical with one error (replacing the last s with an a), but mathe<matics> does not match mathematical no matter how many errors are allowed. (This option is buggy at the moment.)

Regular expressions

Since the index is word based, a regular expression must match words that appear in the index for glimpse to find it. Glimpse first strips the regular expression from all non-alphabetic characters, and searches the index for all remaining words. It then applies the regular expression matching algorithm to the files found in the index. For example, glimpse 'abc.*xyz' will search the index for all files that contain both 'abc' and 'xyz', and then search directly for 'abc.*xyz' in those files. (If you use glimpse -w 'abc.*xyz', then 'abcxyz' will not be found, because glimpse will think that abc and xyz need to be matches to whole words.) The syntax of regular expressions in glimpse is in general the same as that for agrep. The union operation `|', Kleene closure `*', and parentheses () are all supported. Currently '+' is not supported. Regular expressions are currently limited to approximately 30 characters (generally excluding meta characters). The maximal number of errors for regular expressions that use '*' or '|' is 4.

Limitations

The index of glimpse is word based. A pattern that contains more than one word cannot be found in the index. The way glimpse overcomes this weakness is by splitting any multi-word pattern into its set of words and looking for all of them in the index. For example, 'linear programming' will first consult the index to find all files containing both linear and programming, and then apply agrep to find the combined pattern. This is usually an effective solution, but it can be slow for cases where both words are very common, but their combination is not.

As was mentioned in the section on Patterns above, some characters serve as meta characters for glimpse and need to be preceded by '\\' to search for them. The most common examples are the characters '.' (which stands for a wild card), and '*' (the Kleene closure). So, "glimpse ab.de" will match abcde, but "glimpse ab\\.de" will not, and "glimpse ab*de" will not match ab*de, but "glimpse ab\\*de" will. The meta character - is translated automatically to a hypen unless it appears between [] (in which case it denotes a range of characters).

There is no size limit for simple patterns and simple patterns within Boolean expressions. More complicated patterns, such as regular expressions, are currently limited to approximately 30 characters. Lines are limited to 1024 characters.