simple-re.r3

Rebol []

;    PHP
;      $string = preg_replace( '/^@(.*?)@$/', '#$1#', $string );
;
;    Perl
;      $string =~ s/^\@(.*?)\@$/#$1#/;
;
;      # or... 
;      $string =~ s/\A\@(.*?)\@\Z/#$1#/;

string: "@one and @two and three@"

; basic parse match
if [parse/all string ["@" some [to "@" skip] end]] [probe string]

; return copy with changes (though here we've copied back to string !!)
parse/all string [
    "@" start: some [to "@" fin: skip] end 
    (string: rejoin ["#" copy/part start fin "#"])
]

probe string

; in place change (changing it bback to what it was)
parse/all string [
    start: "#" some [to "#" fin: skip] end 
    (change start "@" change fin "@")
    ; a DRYer but longer alternative to above line...
    ;(foreach chg compose [(start) (fin)] [change chg "@"])
]

probe string

; => "@one and @two and three@"
; => "#one and @two and three#"
; => "@one and @two and three@"

test-simple-re.r3

Rebol []

some-text: [
    "nothing here"
    "@start but not end"
    "end but not start@"
    "@nope no @this one"
    "@Do this line@"
    "@yes with @three lots of ats@"
]

; print anything that matches /^@(.*?)@$/ (ie. just the last two lines!)
foreach string some-text [
    parse/all string [
        #"@"
        start: some [to #"@" fin: skip]
        end
        (print copy/part start fin)  ; if parsed true then print captured text
    ]
]