If you have only one or limited patter to replace then it can be done easily with sed as explained earlier.

Please follow the steps.

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1) Save thispatternthatpattern in a file called test
2) Now according to you thispatternthatpattern should be converted to this(pattern)that(pattern) .
3) Now type following command
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sed -i 's/patter/(pattern)/g' test
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4) See the o/p using "cat test". It should be as pasted below.

bash-3.1$ cat test
this(pattern)nthat(pattern)n

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Thanks,