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@@ -1,5 +1,9 @@
+[Contents](../Contents) \| [Previous (1.2 A First Program)](02_Hello_world) \| [Next (1.4 Strings)](04_Strings)
+
 # 1.3 Numbers 
 
+This section discusses mathematical calculations.
+
 ### Types of Numbers
 
 Python has 4 types of numbers:
@@ -27,7 +31,7 @@ if d == 0:
     print('d is False')
 ```
 
-*Don't write code like that. It would be odd.*
+*But, don't write code like that. It would be odd.*
 
 ### Integers (int)
 
@@ -47,9 +51,9 @@ Common operations:
 x + y      Add
 x - y      Subtract
 x * y      Multiply
-x / y      Divide
-x // y     Floor Divide
-x % y      Modulo
+x / y      Divide (produces a float)
+x // y     Floor Divide (produces an integer)
+x % y      Modulo (remainder)
 x ** y     Power
 x << n     Bit shift left  
 x >> n     Bit shift right
@@ -165,12 +169,18 @@ Try it out.
 
 ## Exercises
 
+Reminder: These exercises assume you are working in the `practical-python/Work` directory. Look
+for the file `mortgage.py`. 
+
 ### Exercise 1.7: Dave's mortgage
 
-Dave has decided to take out a 30-year fixed rate mortgage of $500,000 with Guido’s Mortgage, Stock Investment, and Bitcoin trading corporation.
-The interest rate is 5% and the monthly payment is $2684.11.
+Dave has decided to take out a 30-year fixed rate mortgage of $500,000
+with Guido’s Mortgage, Stock Investment, and Bitcoin trading
+corporation.  The interest rate is 5% and the monthly payment is
+$2684.11.
 
-Here is a program that calculates the total amount that Dave will have to pay over the life of the mortgage:
+Here is a program that calculates the total amount that Dave will have
+to pay over the life of the mortgage:
 
 ```python
 # mortgage.py
@@ -238,7 +248,7 @@ While you’re at it, fix the program to correct the for overpayment that occurs
 
 `int()` and `float()` can be used to convert numbers.  For example,
 
-```pycon
+```python
 >>> int("123")
 123
 >>> float("1.23")
@@ -248,7 +258,7 @@ While you’re at it, fix the program to correct the for overpayment that occurs
 
 With that in mind, can you explain this behavior?
 
-```pycon
+```python
 >>> bool("False")
 True
 >>>
 
@@ -1,8 +1,12 @@
+[Contents](../Contents) \| [Previous (1.3 Numbers)](03_Numbers) \| [Next (1.5 Lists)](05_Lists)
+
 # 1.4 Strings
 
-### Representing Text
+This section introduces way to work with text.
 
-String are text literals written in programs with quotes.
+### Representing Literal Text
+
+String literals are written in programs with quotes.
 
 ```python
 # Single quote
@@ -20,12 +24,17 @@ look into my eyes, you're under.
 '''
 ```
 
-Triple quotes capture all text enclosed in multiple lines.
+Normally strings may only span a single line. Triple quotes capture all text enclosed across multiple lines
+including all formatting.
+
+There is no difference between using single (') versus double (")
+quotes.  The same type of quote used to start a string must be used to
+terminate it.
 
 ### String escape codes
 
 Escape codes are used to represent control characters and characters that can't be easily typed
-at the keyboard.  Here are some common escape codes:
+directly at the keyboard.  Here are some common escape codes:
 
 ```
 '\n'      Line feed
@@ -38,8 +47,8 @@ at the keyboard.  Here are some common escape codes:
 
 ### String Representation
 
-The characters in a string are Unicode and represent a so-called "code-point".  You can
-specify an exact code-point using the following escape sequences:
+Each character in a string is stored internally as a so-called Unicode "code-point" which is
+an integer.  You can specify an exact code-point value using the following escape sequences:
 
 ```python
 a = '\xf1'          # a = 'ñ'
@@ -54,6 +63,7 @@ available character codes.
 ### String Indexing
 
 Strings work like an array for accessing individual characters. You use an integer index, starting at 0.
+Negative indices specify a position relative to the end of the string.
 
 ```python
 a = 'Hello world'
@@ -62,7 +72,7 @@ c = a[4]          # 'o'
 d = a[-1]         # 'd' (end of string)
 ```
 
-You can also slice or select substrings with `:`.
+You can also slice or select substrings specifying a range of indices with `:`.
 
 ```python
 d = a[:5]     # 'Hello'
@@ -71,6 +81,8 @@ f = a[3:8]    # 'lowo'
 g = a[-5:]    # 'world'
 ```
 
+The character at the ending index is not included.  Missing indices assume the beginning or ending of the string.
+
 ### String operations
 
 Concatenation, length, membership and replication.
@@ -161,7 +173,8 @@ TypeError: 'str' object does not support item assignment
 
 ### String Conversions
 
-Use `str()` to convert any value to a string suitable for printing.
+Use `str()` to convert any value to a string. The result is a string holding the
+same text that would have been produced by the `print()` statement.
 
 ```python
 >>> x = 42
@@ -172,7 +185,7 @@ Use `str()` to convert any value to a string suitable for printing.
 
 ### Byte Strings
 
-A string of 8-bit bytes, commonly encountered with low-level I/O.
+A string of 8-bit bytes, commonly encountered with low-level I/O, is written as follows:
 
 ```python
 data = b'Hello World\r\n'
@@ -201,9 +214,13 @@ text = data.decode('utf-8') # bytes -> text
 data = text.encode('utf-8') # text -> bytes
 ```
 
+The `'utf-8'` argument specifies a character encoding.  Other common
+values include `'ascii'` and `'latin1'`.
+
 ### Raw Strings
 
-Raw strings are string literals with an uninterpreted backslash. They specified by prefixing the initial quote with a lowercase "r".
+Raw strings are string literals with an uninterpreted backslash. They
+are specified by prefixing the initial quote with a lowercase "r".
 
 ```python
 >>> rs = r'c:\newdata\test' # Raw (uninterpreted backslash)
@@ -237,9 +254,9 @@ is covered later.
 
 ## Exercises
 
-In these exercises, you experiment with operations on Python's string type.
-You should do this at the Python interactive prompt where you can easily see the results.
-Important note:
+In these exercises, you'll experiment with operations on Python's
+string type.  You should do this at the Python interactive prompt
+where you can easily see the results.  Important note:
 
 > In exercises where you are supposed to interact with the interpreter,
 > `>>>` is the interpreter prompt that you get when Python wants
@@ -250,7 +267,7 @@ Important note:
 
 Start by defining a string containing a series of stock ticker symbols like this:
 
-```pycon
+```python
 >>> symbols = 'AAPL,IBM,MSFT,YHOO,SCO'
 >>>
 ```
@@ -259,7 +276,7 @@ Start by defining a string containing a series of stock ticker symbols like this
 
 Strings are arrays of characters. Try extracting a few characters:
 
-```pycon
+```python
 >>> symbols[0]
 ?
 >>> symbols[1]
@@ -273,8 +290,6 @@ Strings are arrays of characters. Try extracting a few characters:
 >>>
 ```
 
-### Exercise 1.14: Strings as read-only objects
-
 In Python, strings are read-only.
 
 Verify this by trying to change the first character of `symbols` to a lower-case 'a'.
@@ -287,22 +302,29 @@ TypeError: 'str' object does not support item assignment
 >>> 
 ```
 
-### Exercise 1.15: String concatenation
+### Exercise 1.14: String concatenation
 
 Although string data is read-only, you can always reassign a variable
 to a newly created string.
 
 Try the following statement which concatenates a new symbol "GOOG" to
 the end of `symbols`:
 
-```pycon
+```python
 >>> symbols = symbols + 'GOOG'
 >>> symbols
 'AAPL,IBM,MSFT,YHOO,SCOGOOG'
 >>>
 ```
 
-Oops!  That's not what you wanted. Fix it so that the `symbols` variable holds the value `'HPQ,AAPL,IBM,MSFT,YHOO,SCO,GOOG'`.
+Oops!  That's not what you wanted. Fix it so that the `symbols` variable holds the value `'AAPL,IBM,MSFT,YHOO,SCO,GOOG'`.
+
+```python
+>>> symbols = ?
+>>> symbols
+'AAPL,IBM,MSFT,YHOO,SCO,GOOG'
+>>>
+```
 
 In these examples, it might look like the original string is being
 modified, in an apparent violation of strings being read only.  Not
@@ -311,12 +333,12 @@ time. When the variable name `symbols` is reassigned, it points to the
 newly created string.  Afterwards, the old string is destroyed since
 it's not being used anymore.
 
-### Exercise 1.16: Membership testing (substring testing)
+### Exercise 1.15: Membership testing (substring testing)
 
 Experiment with the `in` operator to check for substrings.  At the
 interactive prompt, try these operations:
 
-```pycon
+```python
 >>> 'IBM' in symbols
 ?
 >>> 'AA' in symbols
@@ -326,13 +348,13 @@ True
 >>>
 ```
 
-*Why did the check for "AA" return `True`?*
+*Why did the check for `'AA'` return `True`?*
 
-### Exercise 1.17: String Methods
+### Exercise 1.16: String Methods
 
 At the Python interactive prompt, try experimenting with some of the string methods.
 
-```pycon
+```python
 >>> symbols.lower()
 ?
 >>> symbols
@@ -342,14 +364,14 @@ At the Python interactive prompt, try experimenting with some of the string meth
 
 Remember, strings are always read-only.  If you want to save the result of an operation, you need to place it in a variable:
 
-```pycon
+```python
 >>> lowersyms = symbols.lower()
 >>>
 ```
 
 Try some more operations:
 
-```pycon
+```python
 >>> symbols.find('MSFT')
 ?
 >>> symbols[13:17]
@@ -364,14 +386,14 @@ Try some more operations:
 >>>
 ```
 
-### Exercise 1.18: f-strings
+### Exercise 1.17: f-strings
 
 Sometimes you want to create a string and embed the values of
 variables into it.
 
 To do that, use an f-string. For example:
 
-```pycon
+```python
 >>> name = 'IBM'
 >>> shares = 100
 >>> price = 91.1
@@ -383,6 +405,31 @@ To do that, use an f-string. For example:
 Modify the `mortgage.py` program from [Exercise 1.10](03_Numbers) to create its output using f-strings.
 Try to make it so that output is nicely aligned.
 
+
+### Exercise 1.18: Regular Expressions
+
+One limitation of the basic string operations is that they don't
+support any kind of advanced pattern matching.  For that, you
+need to turn to Python's `re` module and regular expressions.  
+Regular expression handling is a big topic, but here is a short
+example:
+
+```python
+>>> text = 'Today is 3/27/2018. Tomorrow is 3/28/2018.'
+>>> # Find all occurrences of a date
+>>> import re
+>>> re.findall(r'\d+/\d+/\d+', text)
+['3/27/2018', '3/28/2018']
+>>> # Replace all occurrences of a date with replacement text
+>>> re.sub(r'(\d+)/(\d+)/(\d+)', r'\3-\1-\2', text)
+'Today is 2018-3-27. Tomorrow is 2018-3-28.'
+>>>
+```
+
+For more information about the `re` module, see the official documentation at
+[https://docs.python.org/library/re.html](https://docs.python.org/3/library/re.html).
+
+
 ### Commentary
 
 As you start to experiment with the interpreter, you often want to