$re = '/^\s+$\n^([ \t]+)Summary.*(?:\n\1[ \t]*\S.*)+/m';
$str = '
COMMENT REVIEWED REFSEQ: This record has been curated by NCBI staff. The
reference sequence was derived from AC105339.9 and FJ695193.1.
This sequence is a reference standard in the RefSeqGene project.
Summary: Adaptor protein complex 3 (AP-3 complex) is a
heterotrimeric protein complex involved in the formation of
clathrin-coated synaptic vesicles. The protein encoded by this gene
represents the beta subunit of the neuron-specific AP-3 complex and
was first identified as the target antigen in human paraneoplastic
neurologic disorders. The encoded subunit binds clathrin and is
phosphorylated by a casein kinase-like protein, which mediates
synaptic vesicle coat assembly. Defects in this gene are a cause of
early-onset epileptic encephalopathy. [provided by RefSeq, Feb
2017].
Another paragrph
PRIMARY REFSEQ_SPAN PRIMARY_IDENTIFIER PRIMARY_SPAN COMP
1-35060 AC105339.9 88079-123138
35061-35259 FJ695193.1 1-199 c
35260-57628 AC105339.9 123337-145705
';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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