$re = '/\{([^\{\}]|(?R))*\}/';
$str = 'Wind energy is the kinetic energy of air in motion, also called wind. Total wind energy flowing through an imaginary surface with area A during the time t is:
{\\displaystyle E={\\frac {1}{2}}mv^{2}={\\frac {1}{2}}(Avt\\rho )v^{2}={\\frac {1}{2}}At\\rho v^{3},} E={\\frac {1}{2}}mv^{2}={\\frac {1}{2}}(Avt\\rho )v^{2}={\\frac {1}{2}}At\\rho v^{3},[248]
where ρ is the density of air; v is the wind speed; Avt is the volume of air passing through A (which is considered perpendicular to the direction of the wind); Avtρ is therefore the mass m passing through "A". Note that ½ ρv2 is the kinetic energy of the moving air per unit volume.
Power is energy per unit time, so the wind power incident on A (e.g. equal to the rotor area of a wind turbine) is:
{\\displaystyle P={\\frac {E}{t}}={\\frac {1}{2}}A\\rho v^{3}.} P={\\frac {E}{t}}={\\frac {1}{2}}A\\rho v^{3}.[248]
Wind power in an open air stream is thus proportional to the third power of the wind speed; the available power increases eightfold when the wind speed doubles. Wind turbines for grid electric power therefore need to be especially efficient at greater wind speeds.';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php