Python Expression Evaluator Version 2

I have extended my Python Expression Evaluator a bit, adding the main feature to support regular expressions . Regular expressions need to be typed in between slashes and need some input text to be applied to, that’s why now a second input field has been added to the user interface.

Regular expressions are very well supported in Python ( like in Perl ), thus not much additional code is needed to support those:

 1: ...
 2:     if expr[0] == "/":
 3:         m = re.match("\/(.*)\/",expr)
 4:         if m:
 5:             expr2 =
 6:             m = re.match(expr2,input)
 7:             if m:
 8:                 for i in range(len(input)):
 9:                     if i in range(m.start(0),m.end(0)):
 10:                         char = "<strong>%s</strong>" % input[i]
 11:                     else: char = "%s" % input[i]
 12:                     for j in range(1,len(m.groups())+1):
 13:                         if i in range(m.start(j),m.end(j)):
 14:                             char = "<span class='highlighted'>%s</span>" % char
 15:                     output += char
 17:     else:
 18: ...

The Python code above first checks whether input contains a regular expression ( starting with a forward slash ). The first regular expression evaluation (re.match(…; re is the name of the Python module for regular expression supported to be imported at the beginning of the program ) is to get the regular expression itself in between the two slashes, the second regular expression evaluation actually evaluates that regular expression. m is the name of the object returned by the evaluation, having some useful attributes:

  • contains the matching part of the input
  • m.start(0) contains the starting position of the matching part of the input
  • m.endt(0) contains the ending position of the matching part of the input
  • m.groups() is a list of groups defined in the regular expression to extract part of the input; defined in form of round brackets within the regular expression
  • with n > 0 is content of group n
  • m.start(n) with n > 0 is the starting position of group n
  • m.end(n) with n > 0 is the ending position of group n

Those useful object attributes help to do what I attempt to do with that code: highlight those characters in my input string being in the overall match or in any of the groups: the first case is indicated by a bold font, being contained in a group by yellow background color. Thus my for loop

 1: for i in range(len(input)):

iterates over the input provided character by character.Then I analyze whether that character is contained in the overall match to put some HTML “strong” tags around it. Then I analyze for each group returned …

 1: for j in range(1,len(m.groups())+1):

… whether the character is contained in a group and give it a yellow background in that case. To do this I use a class called “highlighted” defined in my CSS file for this little application:

 1: .highlighted { background: yellow; }

And here it is: version 2 of my Python Expression Evaluator supporting regular expressions

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