Question: I need to find a regular expression, that ignores certain characters for usage in the bib2bib tool.
For example: I need to find any occurence of the word "muller". But also a string like
 ''Hello, my name is Michael M\"uller, how are you?''
or
 ''There is M\"{u}ller''

should be found.

Edit: I need this to work not only for "muller" but dynamically for every word.

Answer: 
If you want to remove things like `\"` and `\"{` and `}`, you will have to preprocess your input file with a tool like `sed` before feeding it into `bib2bib`. 

Example:

```
 sed -e 's/\\"\{\|\\"\|\}// input.bib > input.bib.preprocessed

```

Or to specifically convert things like `\"{u}` into `u`:

```
 sed -e 's/\\"{\(.\)}/\1/' -e 's/\\"//' input.bib > input.bib.preprocessed

```
You haven't mentioned in what way you have your data available. To remove lines containing the posted patterns you can use `grep`:

```
grep -v -E '(muller|M\\"uller|M\\"{u}ller)'

```

(Note that the `\` needs another escape.) To match the inverse, lines with the given patterns, omit the `-v`.

To define the regular expressions in a file use `grep`'s option `-f`, as in:

```
grep -v -E -f file-with-regexps

```

It expects one regexp per line in that file.