13.5.1. Using pattern parsers

Pattern parsers attempt to parse a part of the message using rules specific to the type of the parser. Parsers are enclosed between @ characters. The syntax of parsers is the following:

  • a beginning @ character,

  • the type of the parser written in capitals,

  • optionally a name,

  • parameters of the parser, if any, and

  • a closing @ character.

Example 13.18. Pattern parser syntax

A simple parser:

@STRING@

A named parser:

@STRING:myparser_name@

A named parser with a parameter:

@STRING:myparser_name:*@

A parser with a parameter, but without a name:

@STRING::*@

Patterns and literals can be mixed together. For example, to parse a message that begins with the Host: string followed by an IP address (for example, Host: 192.168.1.1), the following pattern can be used: Host:@IPv4@.

Note

Note that using parsers is a CPU-intensive operation. Use the ESTRING and QSTRING parsers whenever possible, as these can be processed much faster than the other parsers.

Example 13.19. Using the STRING and ESTRING parsers

For example, look at the following message: user=joe96 group=somegroup.

  • @STRING:mytext:@ parses only to the first non-alphanumeric character (=), parsing only user, so the value of the ${mytext} macro will be user

  • @STRING:mytext:=@ parses the equation mark as well, and proceeds to the next non-alphanumeric character (the whitespace), resulting in user=joe96

  • @STRING:mytext:= @ will parse the whitespace as well, and proceed to the next non-alphanumeric non-equation mark non-whitespace character, resulting in user=joe96 group=somegroup

Of course, usually it is better to parse the different values separately, like this: "user=@STRING:user@ group=@STRING:group@".

If the username or the group may contain non-alphanumeric characters, you can either include these in the second parameter of the parser (as shown at the beginning of this example), or use an ESTRING parser to parse the message till the next whitespace: "user=@ESTRING:user: @group=@ESTRING:group: @".