Difference Between '?' and '*' in regular expressions
flex-lexer, lex, regex
Solution
If you want to match 0, 1, 2, 3, 4, 5, 6, or more occurrences, use `*`.
If you only want to match 0 or 1 occurrences, use `?`.
For instance, consider this text: `"________starts with whitespace"`
If I want to match all of the underscores at the beginning of that text, but I don't want to require that they be there (they're optional), I'd use `_*`.
In contrast, if I to just match an optional single `+` in (say) `"+44 20 1234 5678"`, I'd use `\+?` (a literal `+` with the `?` after it). That will only match the single `+` or nothing, it would not match multiple `+` characters.
Problem
I am learning regular expressions to use them in lex program. I've Seen here that, in Regular Expressions: `'*' matches 0 or more occurances of pattern` `'?' matches 0 or 1 occurance of the pattern` By this I'm kinda Confused. I mean: - if we can match 0 or more by '*' then why should we use '?' meta character? - We define Float as : `FL [0-9]*"."[0-9]+` - Can we define it as : `FL [0-9]?"."[0-9]+` for numbers like 0.999 or .999 etc (ie, Number with only one digit before radix point `.` )? Can Any one please explain this? Thanking you in advance :).