Note that bracket style opening and closing delimiters aren't a 100% problem-free solution, as they need to be escaped when they aren't in matching pairs within the expression. That mismatch can happen when they appear inside character classes [...], as most meta-characters lose their special meaning. Consider these examples:<?php  preg_match('{[{]}', ''); // Warning: preg_match(): No ending matching delimiter '}'  preg_match('{[}]}', ''); // Warning: preg_match(): Unknown modifier ']'  preg_match('{[}{]}', ''); // Warning: preg_match(): Unknown modifier ']'?>Escaping them solves it:<?php  preg_match('{[\{]}', ''); // OK  preg_match('{[}]}', ''); // OK  preg_match('{[\}\{]}', ''); // OK?>

preg_match('{[}]}', ''); // Warning: preg_match(): Unknown modifier ']'preg_match('{[\}]}', ''); // OK

Note that angle brackets `<>` shouldn't be used as delimiters whenever you will have to invoke advanced clusters like atomic groups or lookbehinds because their including angle bracket doesn't come in pair and escaping doesn't help either.