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[A-z] will match ASCII characters in the range from A to z , while [a-zA-Z] will match ASCII characters in the range from A to Z and in the range from a to z . At first glance, this might seem equivalent -- however, if you look at this table of ASCII characters, you'll see that A-z includes several other characters.
[a-zA-Z]* matches zero or more upper- or lower-case letters in a row. ^[a-zA-Z]+$ matches a string that STARTS with one-or more upper- or lower-case letters and also ends with it. Meaning, the only thing in your string is upper- or lower-case letters.
?= is a positive lookahead, a type of zero-width assertion. What it's saying is that the captured match must be followed by whatever is within the parentheses but that part isn't captured. Your example means the match needs to be followed by zero or more characters and then a digit (but again that part isn't captured).
Yes, the first means "match all strings that start with a letter", the second means "match all strings that contain a non-letter". The caret ("^") is used in two different ways, one to signal the start of the text, one to negate a character match inside square brackets.
^[a-zA-Z] means any a-z or A-Z at the start of a line
[^a-zA-Z] means any character that IS NOT a-z OR A-Z
There is a difference.
When the ^ character appears outside of [] matches the beginning of the line (or string). When the ^ character appears inside the [], it matches any character not appearing inside the [].
^ outside of the character class ("[a-zA-Z]") notes that it is the "begins with" operator.
^ inside of the character negates the specified class.
So, "^[a-zA-Z]" translates to "begins with character from a-z or A-Z", and "[^a-zA-Z]" translates to "is not either a-z or A-Z"
Here's a quick reference: http://www.regular-expressions.info/reference.html
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