CSC1018F: Regular Expressions

1
CSC1018FRegular Expressions

• Diving into Python Ch. 7
• Number Systems

2
Lecture Outline

• Recap of OO Python week 3
• Regular Expressions
• Standard
• Verbose
• Number Systems
• Binary, decimal, hexadecimal

3
Recap of OO Python

• Object Orientation
• Module importing
• Defining, initializing and instantiating Classes
• Class attributes
• Class methods
• Exceptions
• File Handling
• Opening, reading, writing and closing

4
Intro to Regular Expressions

• Regular expressions are a powerful means for
  parsing text to identify complex patterns of
  characters
• Standard string methods (find, replace, split)
  can be insufficient in complex cases
• But regular expressions can be complicated and
  difficult to read so avoid them if string methods
  will do the job
• Read regular expressions from left to right
• Usage
• Import re regular expression functionality in
  re module
• Re.sub(regexpr, repstr, inputstr) typical
  search replace

5
Format of Regular Expressions

• Syntax
• - end of string marker
• - start of string marker
• \b - word boundary marker (to avoid backslash
  escapes use a raw string - r"stringcontents")
• ? - optional match to a single character
• (ABC) - indicates mutually exclusive options A,
  B and C
• Examples
• re.sub(r"\bROAD", "RD.", addr)
• addr 60 BROAD ROAD ? 60 BROAD RD.
• re.search(r"(abc) -", question)
• question a - how are you? ? ltSRE_Match object gt

6
Further Syntax

• Pn, m syntax
• Deals with repeating patterns
• Read as pattern P appears at least n times but no
  more than m times
• More syntax
• \d - any numeric digit
• \D - any character except a numeric digit
• - 1 or more
• - 0 or more
• ( ) - to indicate groups
• Examples
• gtgtgt phPat re.compile(r"(\d3)\D(\d7)")
• gtgtgt phPat.search(021 6504058).groups()
• (021, 6504058)

7
Verbose Regular Expressions

• So far only compact regular expressions
• To aid readability we would like to include
  comments and spaces
• Use re.VERBOSE as the last arguments to re
  functions
• Whitespace is ignored
• Comments ( commentstr) are ignored
• Example

pattern """ beginning of string
end of string """
8
Case Study

• Counting 1-10 in roman numerals
• Additive and subtractive combination of I (1),
  V(5), X (10)
• Can have at most 3 of a particular numeral in a
  row

gtgtgt roman r"(I?XIVV?I0,3)" gtgtgt
re.search(roman, "X") lt_sre.SRE_Match object at
0x1e55be0gt gtgtgt re.search(roman,
"VIII") lt_sre.SRE_Match object at 0x1e55ba0gt gtgtgt
re.search(roman, "") lt_sre.SRE_Match object at
0x1e55ce0gt gtgtgt re.search(roman, "IIII")
None True
9
Number Systems

• Decimal (base 10)
• Digits (0-9)
• Each place represents a power of ten
• 172 2100 7101 1102 172
• Binary (base 2)
• Digits (0,1)
• Each place represents a power of two
• 10011 120 121 0 22 0 23 1 24 19
• Hexadecimal (base 16)
• Digits (0-9, A-F)
• A-F represent 10-15
• Each place represents a power of sixteen
• E.g., F7A 10160 7 161 15 162 3962

10
Conversion

• Decimal to others
• Repeatedly divide number by base and populate
  places from right to left with the remainder
• E.g. Dec2Bin 50 / 2 0 25 / 2 1
  12 / 2 0 6 / 2 0 3 / 2 1
  1 / 2 1 0 110010
• Bin2Hex
• Collect binary digits into groups of four and
  convert
• E.g., 111000011111 1110 0001 1111 E1F
• Hex2Bin
• Hexadecimal digits convert into groups of four
  binary digits
• E.g., A7C 1010 0111 1100 101001111100
• Hex is used because
• It is easy to convert to and from binary
• Offers a more compact representation

11
Revision Exercise

• Create a function which will take a date string
  in any one of the following formats
• dd/mm/yyyy or dd/mm/yy
• Other separators (e.g., \, , -) are also
  allowed
• Single figure entries may have the form x or 0x,
  e.g. 3/4/5 or 03/04/05
• dd month yy or yyyy where month may be written in
  full (December) or abbreviated (Dec. or Dec)
• And return it in the format
• dd month(in full) yyyy, e.g. 13 March 2006
• Implement this using regular expressions and also
  implement range checking on dates