I've never got my head around preg_replace formulas

I have a list of airport codes IATA and ICAO format inside a (), for example "(LHR / EGLL)"

How can I extract just the IATA code which is the first 3 letter code?

    One possibility:

    $ php -a
Interactive shell

php > $regex = '#\(\s*([a-zA-Z]{3})\s*/[^\)]+\)#';
php > $string = "(LHR / EGLL)";
php > preg_match_all($regex, $string, $matches);
php > print_r($matches);
Array
(
    [0] => Array
        (
            [0] => (LHR / EGLL)
        )

[1] => Array
    (
        [0] => LHR
    )

)
php > echo $matches[1][0];
LHR

    NogDog $ php -a
    Interactive shell

    Hmm, thanks for this! I really thought I'd tried the shell before. Lots of CLI work, but maybe I'd not ever compiled in --with-readline? Look out Python 😛 😃

      dalecosp Look out Python

      IIRC, the python interactive shell does a better job of auto-indenting nested stuff for you?

          Hmm, dunno about that. I don't know if you have to set something?

          Python 3.6.8 (default, Apr 11 2019, 01:12:23)
[GCC 4.2.1 Compatible FreeBSD Clang 6.0.0 (tags/RELEASE_600/final 326565)] on freebsd11
Type "help", "copyright", "credits" or "license" for more information.
>>> foo = 1
>>> if foo > 0:
... print "Foo has value"
  File "<stdin>", line 2
    print "Foo has value"
        ^
IndentationError: expected an indented block

          I guess maybe it hinted, but didn't actually do it?

            NZ_Kiwis Can you explain the regex to me?

            #        regex delimiter (typically '/', but can be many special chars)
\(       literal opening paren (escaped because of regex meaning)
\s*      0-n white-space chars ('*' is the 0-n repetition indicator)
(        start of capturing sub-pattern (shows up separately in $matches)
[a-zA-Z] any letter
{3}      match exactly 3 of the preceding thing (any letter)
)        end of sub-pattern
\s*      0-n white-space chars
/        literal slash (that's why I didn't use it as the regex delimiter)
[^\)]+   1-n characters that are not a closing paren ('+' is the 1-n indicator)
\)       a closing paren (escaped because of regex meaning) 
#        regex delimiter

                Not sure this would work (because haven't seen all data; it works with your example), but you might also try:

                $iata = trim(strtok($string, "/"), "( ");
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