判断字符串是不是正则表达式

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:rules="[             { required: true, trigger: 'blur', validator: this.checkCanonical },           ]"   checkCanonical (rule, value, callback) {       if (value) {         let isReg = true         try {           isReg = eval(value) instanceof RegExp         } catch (e) {           isReg = false         }         isReg ? callback() : callback(new Error('请输入正确的正则表达式'))       } else {         callback(new Error('请输入正则表达式'))       }     },