Fix issue with RegExp for getting context and replace context in blueprint

[legiar]

Fix issue with getting contex. Javascript always replace backslash.

Also fix issue with replace context parents in content. For example:

project[tasks_attributes][0][assigments_sttributes[new_assigments][file_name]

will be replaced to

project[tasks_attributes][0][assigments_sttributes[0][file_name]

and will replace first child object after post.

[lest]

@legiar Seems like your change cause existing tests to fail. Could you please provide more details about the issue you're fixing? A test application reproducing it will be helpful. Thanks.