Lecture 13 Revision: Regular Expressions (Regex) Cont.

Question 1

In Regex what is the wildcard character?

Answer 1

To match ANY character use a full stop (period) .
* The period character matches ONE character,
which can be any character of any type.

• Example:– Find any three characters:
  r’…’
  – Find any line that contains only one character:
  r’^.$’

Question 2

So how do you match a . (full stop / period character)?

Answer 2

Remember the period or full stop is a special char, so need to use . to escape it.

Question 3

What is REPETITION?

Answer 3

Can use REPETITION to reduce the size of regex patterns.

Example:– Match any word that contains three lowercase letters:
r’\b[a-z][a-z][a-z]\b’

Does not scale well: What if I had said 50 characters?
* Using repetition
r’\b[a-z]{3}\b

The REPETITION is in the curly brackets

Question 4

What is the syntax for using REPETITION?

Answer 4

• There are multiple ways to specify repetition
  – 5 ‘a’ characters
  a{5}

– 5 or more ‘a’ characters.
a{5,}

– between 5 and 7 ‘a’ characters
a{5,7}

– between 3 and 5 lowercase characters
r’\b[a-z]{3,5}\b’

Question 5

What is the special shorthand for repetitions that are used a lot?

Answer 5

• means zero or more occurences. Same as using {0,}

+ means one or mroe occurences. Same as using {1,}

? means zero or one occurence. Same as {0,1}

Need to escape all of these with a back slash in front.

Question 6

What are ALTERNATIVES in regex?

Answer 6

ALTERNATIVES provide a way to match one of several patterns. In Python, you can use the pipe symbol (|) to specify alternatives. The | acts as an “OR” operator between patterns.

Question 7

Alternatives cont.: How could we write a pattern to match cat OR bat?

Answer 7

1. r’cat|bat’ # using alteration
2. r’[cb]at’
   # using a character class
3. r’(c|b)at’ #Best one

Question 8

Explain this regex pattern:

r’\b(hack|crack)(ing|ed)?\b’

Answer 8

\b after a word break

search for either hack or crack

then the (ing|ed)? gives an optional ing OR ed at the end. Because the ing and ed are in brackets, the ? means that this is optional.

Then word break at the end.

So this will match either hack, crack, hacking, cracking, cracked or hacked.

Question 9

So how can we EXTRACT the information we find from regex? I.e. how do we extract the match.

Answer 9

Use match.group()

Question 10

How do we use match.group()?

Answer 10

Use () to match parts of the match. These are stored separately in match.group()

match= re.search(r’([A-Za-z]+), ([0-9]+)’,csv)

The bit in the first brackets is match.group(1), the bit in the second brackets is match.group(2).

name = match.group(1)

number = match.group(2)

Question 11

What is SUBSTITUTION?

Answer 11

• Regular expressions can be used to perform substitution (search & replace) - like find and replace in word.
• Example: Replace occurrences of ‘H’ with ‘h’

text = re.sub(r’H’, r’h’, text)

Question 12

What is the difference between re.search() and re.findall()?

Answer 12

re.search() finds the first occurrence of the pattern.

re.findall() will find all occurrences!