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Python 7 - Regular Expressions - Problem Set

  1. In the file Python_07_nobody.txt find every occurrence of 'Nobody' and print out the position.

  2. In the file Python_07_nobody.txt substitute every occurrence of 'Nobody' with your favorite name and write an output file with that person's name (ex. Michael.txt).

  3. Using pattern matching, find all the header lines in Python_07.fasta. Note that the format for a header in a fasta file is a line that starts with a greater than symbol and is followed by some text (e.g. >seqName description where seqName is the sequence name or identifier. The identifier cannot have spaces in it. The description that follows it can have spaces.)

  4. If a line matches the format of a FASTA header, extract the sequence name and description using sub patterns (groups).

    • Print lines something like this id:"extracted seq name" desc:"extracted description"
  5. Create a FASTA parser, or modify your FASTA parser from the previous problem set, to use regular expressions. Also make sure your parser can deal with a sequence that is split over many lines.

  6. The enzyme ApoI has a restriction site: R^AATTY where R and Y are degenerate nucleotideides. See the IUPAC table to identify the nucleotide possibilities for the R and Y. Write a regular expression to find and print all occurrences of the site in the following sequence Python_07_ApoI.fasta.

  1. Determine the site(s) of the physical cut(s) by ApoI in the above sequence. Print out the sequence with "^" at the cut site.


  • Use sub()
  • Use subpatterns (parentheses and group() ) to find the cut site within the pattern.
  • Example: if the pattern is GACGT^CT the following sequence

we want to display the cut site like this:

  1. Now that you've done your restriction digest, determine the lengths of your fragments and sort them by length (in the same order they would separate on an electrophoresis gel).

Hint: Convert this string:


Into this list:

  1. Download this file: of enzymes and their cut sites to fill a dictionary of enzymes paired with their recognition patterns. Skip the top 10 header lines and be aware of how the columns are delimited. You'll modify this script in the next question.

  2. Write a script which takes two command line arguments: the name of an enzyme and a fasta file with a sequence to be cut. Load a dictionary of enzyme names and cut sites from the code you developed in question 9. If the enzyme is present in the dictionary, and can cut the sequence, print out:

    • the sequence, annotated with cut sites
    • the number of fragments
    • the fragments in their natural order (unsorted)
    • the fragments in sorted order (largest to smallest)