Irule syntax issue : missing a script

Perhaps it's the formatting but which line is 18 please?

It's a problem on formatting.

After each if condition you have to add curly brackets.

if { your condition } {
    What_you_want_to_do
}
else { other_condition } {
    actions
}

It's a problem on formatting.

After each if condition you have to add curly brackets.

if { your condition } {
    What_you_want_to_do
}
else { other_condition } {
    actions
}

I mean with you code :

if {[$fract matches_regex "\d\d"]} {  
    append fract 0  
} elseif {$fract matches_regex {/\d} } {  
    append fract 00
}

 I just test the syntax and it works.
 Remember: 
 Trim the end of each line, indent the beginning for best pratices 
 and write the conditions with curly braces.

 To paste the code on this forum, start each line with {tab} character

when HTTP_REQUEST {

    set secs [clock seconds]
    set msec [clock clicks -milliseconds]
    set base [expr { $secs * 1000 } ]
    set fract [expr { $msec - $base }]

    log local0. "step_1 $fract"

    if {$fract >= 1000 } {
        set diff [expr { $fract / 1000 }]
        incr secs $diff
        incr fract [expr { -1000 * $diff }]
    }

    log local0. "step_2 $fract"

    if {[$fract matches_regex "\d\d"]} {
        append fract 0
    }
    elseif {$fract matches_regex {/\d} } {
        append fract 00
    }

    log local0. "Timestamp with milliseconds is $secs.$fract"
}

I even found a way to do the job i wanted within one single line : set timestamp [format "%d%03d000" $timestamp $fract]

Thanks to a colleague who speaks fluently perl, tcl among others.