EXTENSION LOGIC : 

There are two levels of parameter evaluation done in asterisk in 
extensions.conf.
The first, and most frequently used, is the substitution of variable
references with their values. 

Then there are the evaluations done in $[ .. ]. This will be
discussed below.

___________________________
PARAMETER QUOTING: 
---------------------------

exten => s,5,BackGround,blabla

The parameter (blabla) can be quoted ("blabla"). In this case, a 
comma does not terminate the field. However, the double quotes
will be passed down to the Background command, in this example.

Also, characters special to variable substitution, expression evaluation, etc
(see below), can be quoted. For example, to literally use a $ on the 
string "$1231", quote it with a preceding \. Special characters that must
be quoted to be used, are [ ] $ " \. (to write \ itself, use \\). 

These Double quotes and escapes are evaluated at the level of the
asterisk config file parser. 

Double quotes can also be used inside expressions, as discussed below.


___________________________
VARIABLES: 
---------------------------

Parameter strings can include variables. Variable names are arbitrary strings. 
They are stored in the respective channel structure. 

To set a variable to a particular value, do : 

;exten => 1,2,SetVar,varname=value

You can substitute the value of a variable everywhere using ${variablename}.
For example, to stringwise append $lala to $blabla and store result in $koko, 
do: 

;exten => 1,2,SetVar,koko=${blabla}${lala}

There are also the following special variables: 

${ACCOUNTCODE} 	Account code (if specified)
${CALLERID}	Caller ID
${CALLERIDNAME}	Caller ID Name only
${CALLERIDNUM}	Caller ID Number only
${CALLINGPRES}	PRI Caller ID presentation for incoming calls
${CHANNEL}	Current channel name
${CONTEXT}      Current context
${DATETIME}	Current date time in the format: YYYY-MM-DD_HH:MM:SS
${DNID}         Dialed Number Identifier
${ENUM}		Result of application EnumLookup
${EPOCH}	Current unix style epoch
${EXTEN}	Current extension
${ENV(VAR)}	Environmental variable VAR
${HANGUPCAUSE}	Asterisk hangup cause
${INVALID_EXTEN}The invalid called extension (used in the "i" extension)
${LANGUAGE}	Current language
${LEN(VAR)}	String length of VAR (integer)
${MEETMESECS} 	Number of seconds a user participated in a MeetMe conference
${PRIORITY}	Current priority
${RDNIS}        Redirected Dial Number ID Service
${SIPCALLID} 	SIP Call-ID: header verbatim (for logging or CDR matching)
${SIPDOMAIN}    SIP destination domain of an inbound call (if appropriate)
${SIPUSERAGENT} SIP user agent 
${TIMESTAMP}	Current date time in the format: YYYYMMDD-HHMMSS
${TXTCIDNAME}	Result of application TXTCIDName
${UNIQUEID}	Current call unique identifier

The dial() application sets the following variables:

${DIALEDPEERNAME} Dialed peer name
${DIALEDPEERNUMBER} Dialed peer number
${DIALEDTIME}   Time for the call (seconds)
${ANSWEREDTIME} Time from dial to answer (seconds)
${DIALSTATUS}   Status of the call, one of:
                CHANUNAVAIL | CONGESTION | BUSY | NOANSWER | ANSWER | CANCEL

There are two reference modes - reference by value and reference by name. 
To refer to a variable with its name (as an argument to a function that 
requires a variable), just write the name. To refer to the variable's value, 
enclose it inside ${}. For example, SetVar takes as the first argument 
(before the =) a variable name, so: 

;exten => 1,2,SetVar,koko=lala
;exten => 1,3,SetVar,${koko}=blabla

stores to the variable "koko" the value "lala" and to variable "lala" the 
value "blabla". 

In fact, everything contained ${here} is just replaced with the value of 
the variable "here". 

___________________________
EXPRESSIONS: 
---------------------------

Everything contained inside a bracket pair prefixed by a $ (like $[this]) is 
considered as an expression and it is evaluated. Evaluation works similar to 
(but is done on a later stage than) variable substitution: the expression 
(including the square brackets) is replaced by the result of the expression 
evaluation. The arguments and operands of the expression MUST BE separated 
by at least one space. 


For example, after the sequence: 

exten => 1,1,SetVar,"lala=$[1 + 2]";
exten => 1,2,SetVar,"koko=$[2 * ${lala}]";

the value of variable koko is "6".

And, further:

exten => 1,1,SetVar,"lala=$[1+2]";

will not work as you might have expected. Since all the chars in the single 
token "1+2" are not numbers, it will be evaluated as the string "1+2". Again,
please do not forget, that this is a very simple parsing engine, and it
uses a space (at least one), to separate "tokens".

and, further:

exten => 1,1,SetVar,"lala=$[  1 +    2   ]";

will parse as intended. Extra spaces are ignored.

___________________________
SPACES INSIDE VARIABLE
---------------------------
If the variable being evaluated contains spaces, there can be problems.

For these cases, double quotes around text that may contain spaces
will force the surrounded text to be evaluated as a single token.
The double quotes will be counted as part of that lexical token.

As an example:

exten => s,6,GotoIf($[ "${CALLERIDNAME}" : "Privacy Manager" ]?callerid-liar|s|1:s|7)

The variable CALLERIDNAME could evaluate to "DELOREAN MOTORS" (with a space)
but the above will evaluate to:

"DELOREAN MOTORS" : "Privacy Manager"

and will evaluate to 0.

The above without double quotes would have evaluated to:

DELOREAN MOTORS : Privacy Manager

and will result in syntax errors, because token DELOREAN is immediately
followed by token MOTORS and the expression parser will not know how to 
evaluate this expression.

_____________________
OPERATORS
---------------------
Operators are listed below in order of increasing precedence.  Operators
with equal precedence are grouped within { } symbols.

     expr1 | expr2
             Return the evaluation of expr1 if it is neither an empty string
             nor zero; otherwise, returns the evaluation of expr2.

     expr1 & expr2
             Return the evaluation of expr1 if neither expression evaluates to
             an empty string or zero; otherwise, returns zero.

     expr1 {=, >, >=, <, <=, !=} expr2
             Return the results of integer comparison if both arguments are
             integers; otherwise, returns the results of string comparison
             using the locale-specific collation sequence.  The result of each
             comparison is 1 if the specified relation is true, or 0 if the
             relation is false.

     expr1 {+, -} expr2
             Return the results of addition or subtraction of integer-valued
             arguments.

     expr1 {*, /, %} expr2
             Return the results of multiplication, integer division, or
             remainder of integer-valued arguments.

     expr1 : expr2
             The `:' operator matches expr1 against expr2, which must be a
             regular expression.  The regular expression is anchored to the
             beginning of  the string with an implicit `^'.

             If the match succeeds and the pattern contains at least one regu-
             lar expression subexpression `\(...\)', the string correspond-
             ing to `\1' is returned; otherwise the matching operator
             returns the number of characters matched.  If the match fails and
             the pattern contains a regular expression subexpression the null
             string is returned; otherwise 0.

Parentheses are used for grouping in the usual manner.

The parser must be parsed with bison (bison is REQUIRED - yacc cannot 
produce pure parsers, which are reentrant) 

___________________________
CONDITIONALS
---------------------------

There is one conditional operator - the conditional goto : 

;exten => 1,2,gotoif,condition?label1:label2

If condition is true go to label1, else go to label2. Labels are interpreted
exactly as in the normal goto command.

"condition" is just a string. If the string is empty or "0", the condition
is considered to be false, if it's anything else, the condition is true. 
This is designed to be used together with the expression syntax described 
above, eg : 

exten => 1,2,gotoif,$[${CALLERID} = 123456]?2|1:3|1


Example of use : 

exten => s,2,SetVar,"vara=1"
exten => s,3,SetVar,"varb=$[${vara} + 2]"
exten => s,4,SetVar,"varc=$[${varb} * 2]"
exten => s,5,GotoIf,"$[${varc} = 6]?99|1:s|6";

___________________________
PARSE ERRORS
---------------------------

Syntax errors are now output with 3 lines.

If the extensions.conf file contains a line like:

exten => s,6,GotoIf($[ "${CALLERIDNUM}"  = "3071234567" & "${CALLERIDNAME}" : "Privacy Manager" ]?callerid-liar|s|1:s|7)

You may see an error in /var/log/asterisk/messages like this:

May  3 15:58:53 WARNING[1234455344]: ast_yyerror(): syntax error: parse error; Input:
 "3072312154"  : "3071234567" & & "Steves Extension" : "Privacy Manager"
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
                                  ^

The first line shows the string passed to the expression parser. This
string is the result of the variable replacements, etc. This way, you
can see the actual string that went into the parser.

The second line usually shows a string of '^' chars, that show what's
been legally parsed so far.

And the third line shows where the parser was (lookahead token lexing,
etc), when the parse hit the rocks. A single '^' here. The error is
going to be somewhere between the last '^' on the second line, and the
'^' on the third line. That's right, in the example above, there are two
'&' chars, separated by a space, and this is a definite no-no!


___________________________
NULL STRINGS
---------------------------

Testing to see if a string is null can be done in one of two different ways:

exten => _XX.,1,GotoIf($["${calledid}" != ""]?3) 

exten => _XX.,1,GotoIf($[foo${calledid} != foo]?3) 


The second example above is the way suggested by the WIKI. It will 
work as long as there are no spaces in the evaluated value.

The first way should work in all cases, and indeed, might now
be the safest way to handle this situation.

___________________________
WARNING
---------------------------

If you need to do complicated things with strings, asterisk expressions
is most likely NOT the best way to go about it. AGI scripts are an
excellent option to this need, and make available the full power of
whatever language you desire, be it Perl, C, C++, Cobol, RPG, Java,
Snobol, PL/I, Scheme, Common Lisp, Shell scripts, Tcl, Forth, Modula,
Pascal, APL, assembler, etc.

