BlockedUnblock FollowFollowing Web frameworks guy. They are presented with examples below in the first part of the article. In the second part this article explains the very basic math behind each of them.
Previous lesson Index Next lesson Regular expressions provide a very powerful method of defining a pattern, but they are a bit awkward to understand and to use properly. So let us examine some more examples in detail.
We start with a simple yet non-trivial example: We only consider numbers like 1. How do we design our regular expression for this problem?
By examining typical examples of the strings we want to match: A pattern is beginning to emerge: A number can have zero or more digits in front of a single period. A number may not contain a period at all.
So, revise the previous expression to: Try the expression with a bunch of examples like the ones above and see if the proper ones match and the others do not. Try to make it look nicer, before we start off testing it. So, we could settle for: Or we could decide that we want to capture the digits before and after the period for special processing: You see, there is a problem with the above pattern: Our pattern can match an empty string!
But more surprisingly, the strings "" and "A1B2" will be accepted too! Because the pattern can start anywhere in the string, so it would match the substrings "-1" and "1" respectively!
We need to reconsider our pattern - it is too simple, too permissive: The character before a minus or a plus, if there is any, can not be another digit, a period or a minus or plus.
Let us make it a space or a tab or the beginning of the string: Any sequence of digits before the period if there is one is allowed: There may be zero digits in front of the period, but then there must be at least one digit behind it: Each set of parentheses also defines a substring and this can be put into a separate variable: To identify these substrings: If we put it to the test: Does not contain a valid number So our pattern correctly accepts the strings we intended to be recognised as numbers and rejects the others.
Let us turn to some other patterns now: Text enclosed in a string: This is "quoted text". Suppose we do not know the enclosing character it can be " or '. You can use this technique to see if a word occurs twice in the same line of text: Suppose you need to check the parentheses in some mathematical expression: A simple check is counting the open and close parentheses: Use the return value of [regexp] to count the number of parenthesesAll the Digits.
Age 7 to 11 The whole calculation uses each of the digits $0 - 9$ once and once only.
The $4$-figure number contains three consecutive numbers, which are not in order. The third digit is the sum of two of the consecutive numbers. The second and fourth digits are also consecutive numbers. Can you replace the stars in the. The following examples illustrate the use and construction of simple regular expressions.
Each example includes the type of text to match, one or more regular expressions that match that text, and notes that explain the use of the special characters and formatting. Replace all letters with a digit and solve the equations (letters vs digit). Replace all of the ten distinct letter s in the scheme with a single digit (the letters are distinct but not the digit) You must not assign different digits to different letters.
Chapter 6 Strings A string is a sequence A string is a sequence of characters. You can access the characters one at a time with the bracket operator: >>> fruit = 'banana' >>> letter = fruit The second statement extracts the character at index position 1 from the fruit variable and assigns it to letter variable.
The expression in brackets is called an index. Jun 20, · Word: Replace and reformat text inside square brackets using wildcards June 20, To deal with the range 1,,,,, we must add up all digits from 1 to 7, and add on 7 times the sum of the other digits, to deal with all numbers greater .