CompSci 101 : Fall 2013

Reasoning about Code

Regex Questions

Start by snarfing the code from the class website. Alternatively, you can browse code here.

Think about the following pieces of code and try them on paper rather than simply typing them into the interpreter. After your group is confident in its answer, try your regular expression on a list of dictionary words using the program you downloaded.

What five letter words end with the letter 'd'?
```
\w{4}d$
```

The regular expression [aeiou]$ indicates there are 6988 words that end in a vowel. What words start and end with a vowel?
```
^[aeiou]\w*[aeiou]$
```

What words start and end with the same vowel, like amoeba?
```
^([aeiou])\w*\1$
```

The words obsequious and pharmacopoeia each contain four vowels in a row. What words contain four consecutive vowels?
```
  
```
The word maintaining contains the two letter sequence "in" repeated three times — in genomics this could be called a triple-repeat. What words contain the two letter sequence "in" repeated three times (letters may appear between the repeats)?
The word contentment contains the two letter sequence "nt" repeated three times. What words contain some two letter sequence repeated three times? For example, both contentment and maintaining are among such words (letters may appear between the repeats).
The word interpreter contains the two letter sequence "er" followed by the sequence reversed "re" followed by the original two letter sequence "er". What words contain some two letter sequence followed by the sequence reversed followed by the original sequence (letters may appear between the repeats)?
You may have seen the word puzzle "what English word contains three doubled-letters in a row?" to which the answer is "bookkeeper" with double-o, double-k, double-e in a row. If we allow letters to appear between the doubled-letters we get more words, like unsuccessfully which has a double-c, double-s, and double-l. What words contain three doubled letters appearing anywhere in the word, not necessarily adjacent to each other?
The regular expression ((\w)\w\2)\w*\1 matches the 12 words below (and only these words). Explain why these words match, how the regular expression works. Note that the expression (\w) is the second tagged expression since the first left parenthesis begins the first tagged expression — you count tagged expressions by the order in which the left-parenthesis appears in reading left-to-right.
```
amalgamate
amalgamated
amalgamates
amalgamatating
amalgamatation
assesses
dereference
monotonous
monotonously
monotonousness
possesses
sensitivities
```
The regular expression ((\w)\w\2){3} matches the two words below (and only these two words). Explain why these words match, i.e., how the regular expression works.
```
agamemnon
precipitateness
```

Write a word-puzzle question for the lab section to answer (you must have a solved it first!). We will pose your puzzles to the class as the last part of this lab.