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JavaScript
var tag = '[tag_share d attr="val" attr2="val"]';
// or var tag = '[tag_share]';
var match = tag.match(/.*\[tag_(.*?)\W.*/)[1];
alert(match);
var tag = '[tag_share d attr="val" attr2="val"]';
// or var tag = '[tag_share]';
var match = tag.match(/.*\[tag_(.*?)\W.*/)[1];
alert(match);