Python Beginner Regular Expressions

Question

what is the use of the = in the regex (?=.*?[A-Z]) and why are the ? and * in front of the [a-z] because I saw in a book that they should appear behind the word or expression they should take effect on and why the two ?

Answer 1

This whole RegEx

(?=.*?[A-Z])

is called a lookahead assertion, a kind of lookarounds.

It consists of three items:

(?=   )
.*?
[A-Z]

The first one is the syntax for a lookahead assertion. The pattern would come in the brackets, after the initial ?=.

The second one is a dot that matches any character, with a repetition modifier *?, where the asserisk means "zero or more matches" and the question mark means "match as few as possible" instead of being greedy.

The third one I suppose you know it.

A lookaround assertion restricts the surrounding of a pattern without matching (capturing) extra things. For example:

a(?=b)

will match the letter a in ab, but not ac. Note it only matches the letter a, and the letter b is only a restriction about where the letter a should be matched. Whereas a(b) matches both letters in ab and captures the latter.