# Filtering Messages

When selecting messages for display in the Filtered Messages window, note that the overall filtering specified by the Filter menu does not apply to the Filtered Messages window; only the filters specified in the window will have any effect on the messages seen. Note also that only APRS Messages (prefix ":", as specified in Chapter 14 of the APRS 1.01 specification) will be seen in the window.

There are three separate filters, of which not all have to be used; any message to be displayed and logged must pass all defined filters:

- sent from an approved sending station (whitelist).
- sent to a desired destination addressee (note this is not the AX.25 destination address in the RF frame, which is usually a sending station type alias, but the addressee field just prior to the Message body).
- having the Message body text matching a desired regular expression pattern.

Regular expressions are defined as the Java version of the POSIX.2 specification. The full capabilities defined by the java.util.regex.Pattern class may be used.

#### Summary of regular-expression constructs

Construct | Matches
--- | ---
Characters
x | The character x
`\\` | The backslash character
`\0` n | The character with octal value `0` n (0 `<=` n `<=` 7)
`\0` nn | The character with octal value `0` nn (0 `<=` n `<=` 7)
`\0` mnn | The character with octal value `0` mnn (0 `<=` m `<=` 3, 0 `<=` n `<=` 7)
`\x` hh | The character with hexadecimal value `0x` hh
`\u` hhhh | The character with hexadecimal value `0x` hhhh
`\t` | The tab character (`'\u0009'`)
`\n` | The newline (line feed) character (`'\u000A'`)
`\r` | The carriage-return character (`'\u000D'`)
`\f` | The form-feed character (`'\u000C'`)
`\a` | The alert (bell) character (`'\u0007'`)
`\e` | The escape character (`'\u001B'`)
`\c` x | The control character corresponding to x
Character classes
`[abc]` | `a`, `b`, or `c` (simple class)
`[^abc]` | Any character except `a`, `b`, or `c` (negation)
`[a-zA-Z]` | `a` through `z` or `A` through `Z`, inclusive (range)
`[a-d[m-p]]` | `a` through `d`, or `m` through `p`: `[a-dm-p]` (union)
`[a-z&&[def]]` | `d`, `e`, or `f` (intersection)
`[a-z&&[^bc]]` | `a` through `z`, except for `b` and `c`: `[ad-z]` (subtraction)
`[a-z&&[^m-p]]` | `a` through `z`, and not `m` through `p`: `[a-lq-z]`(subtraction)
Predefined character classes
`.` | Any character (may or may not match line terminators)
`\d` | A digit: `[0-9]`
`\D` | A non-digit: `[^0-9]`
`\s` | A whitespace character: `[ \t\n\x0B\f\r]`
`\S` | A non-whitespace character: `[^\s]`
`\w` | A word character: `[a-zA-Z_0-9]`
`\W` | A non-word character: `[^\w]`
POSIX character classes (US-ASCII only)
`\p{Lower}` | A lower-case alphabetic character: `[a-z]`
`\p{Upper}` | An upper-case alphabetic character:`[A-Z]`
`\p{ASCII}` | All ASCII:`[\x00-\x7F]`
`\p{Alpha}` | An alphabetic character:`[\p{Lower}\p{Upper}]`
`\p{Digit}` | A decimal digit: `[0-9]`
`\p{Alnum}` | An alphanumeric character:`[\p{Alpha}\p{Digit}]`
`\p{Punct}` | Punctuation: One of `!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~`
`\p{Graph}` | A visible character: `[\p{Alnum}\p{Punct}]`
`\p{Print}` | A printable character: `[\p{Graph}\x20]`
`\p{Blank}` | A space or a tab: `[ \t]`
`\p{Cntrl}` | A control character: `[\x00-\x1F\x7F]`
`\p{XDigit}` | A hexadecimal digit: `[0-9a-fA-F]`
`\p{Space}` | A whitespace character: `[ \t\n\x0B\f\r]`
java.lang.Character classes (simple java character type)
`\p{javaLowerCase}` | Equivalent to java.lang.Character.isLowerCase()
`\p{javaUpperCase}` | Equivalent to java.lang.Character.isUpperCase()
`\p{javaWhitespace}` | Equivalent to java.lang.Character.isWhitespace()
`\p{javaMirrored}` | Equivalent to java.lang.Character.isMirrored()
Classes for Unicode blocks and categories
`\p{InGreek}` | A character in the Greek block
`\p{Lu}` | An uppercase letter
`\p{Sc}` | A currency symbol
`\P{InGreek}` | Any character except one in the Greek block (negation)
`[\p{L}&&[^\p{Lu}]]` | Any letter except an uppercase letter (subtraction)
Boundary matchers
`^` | The beginning of a line
`$` | The end of a line
`\b` | A word boundary
`\B` | A non-word boundary
`\A` | The beginning of the input
`\G` | The end of the previous match
`\Z` | The end of the input but for the final terminator, if any
`\z` | The end of the input
Greedy quantifiers
X`?` | X, once or not at all
X`*` | X, zero or more times
X`+` | X, one or more times
X`{`n`}` | X, exactly n times
X`{`n`,}` | X, at least n times
X`{`n`,`m`}` | X, at least n but not more than m times
Reluctant quantifiers
X`??` | X, once or not at all
X`*?` | X, zero or more times
X`+?` | X, one or more times
X`{`n`}?` | X, exactly n times
X`{`n`,}?` | X, at least n times
X`{`n`,`m`}?` | X, at least n but not more than m times
Possessive quantifiers
X`?+` | X, once or not at all
X`*+` | X, zero or more times
X`++` | X, one or more times
X`{`n`}+` | X, exactly n times
X`{`n`,}+` | X, at least n times
X`{`n`,`m`}+` | X, at least n but not more than m times
Logical operators
XY | X followed by Y
X`|`Y | Either X or Y
`(`X`)` | X, as a capturing group
Back references
`\`n | Whatever the nth capturing group matched
Quotation
`\` | Nothing, but quotes the following character
`\Q` | Nothing, but quotes all characters until `\E`
`\E` | Nothing, but ends quoting started by `\Q`
Special constructs (non-capturing)
`(?:`X`)` | X, as a non-capturing group
`(?idmsux-idmsux)` | Nothing, but turns match flags i d m s u x on - off
`(?idmsux-idmsux:`X`)` | X, as a non-capturing group with the given flags i d m s u x on - off
`(?=`X`)` | X, via zero-width positive lookahead
`(?!`X`)` | X, via zero-width negative lookahead
`(?<=`X`)` | X, via zero-width positive lookbehind
`(?<!`X`)` | X, via zero-width negative lookbehind
`(?>`X`)` | X, as an independent, non-capturing group

#### Backslashes, escapes, and quoting

The backslash character (`'\'`) serves to introduce escaped constructs, as defined in the table above, as well as to quote characters that otherwise would be interpreted as unescaped constructs. Thus the expression `\\` matches a single backslash and `\{` matches a left brace.

It is an error to use a backslash prior to any alphabetic character that does not denote an escaped construct; these are reserved for future extensions to the regular-expression language. A backslash may be used prior to a non-alphabetic character regardless of whether that character is part of an unescaped construct.

As soon as a filter is changed, the changes will apply to all Messages arriving after the change; it will not retroactively affect Messages already logged.