CODEWITHSUNDEEP
regex
Ahmad
Ahmad

Reputation: 13406

Make regular expression not match empty string?

I have regex which works fine in my application, but it matches an empty string too, i.e. no error occurs when the input is empty. How do I modify this regex so that it will not match an empty string ? Note that I DON'T want to change any other functionality of this regex.

This is the regex which I'm using: ^([0-9\(\)\/\+ \-]*)$

I don't know a lot about regex formulation myself, which is why I'm asking. I have searched for an answer, but couldn't find a direct one. Closest I got to was this: regular expression for anything but an empty string in c#, but that doesn't really work for me ..

Upvotes: 15

Views: 35238

Answers (4)

lejlot
lejlot

Reputation: 66775

Replace * with +, as * means "0 or more occurrences", while + means "at least one occurrence"

Upvotes: 20

Wiktor Stribiżew
Wiktor Stribiżew

Reputation: 626728

There are a lot of pattern types that can match empty strings. The OP regex belongs to an ^.*$ type, and it is easy to modify it to prevent empty string matching by replacing * (= {0,}) quantifier (meaning zero or more) with the + (= {1,}) quantifier (meaning one or more), as has already been mentioned in the posts here.

There are other pattern types matching empty strings, and it is not always obvious how to prevent them from matching empty strings.

Here are a few of those patterns with solutions:

[^"\\]*(?:\\.[^"\\]*)* ⇒ (?:[^"\\]|\\.)+

abc||def ⇒ abc|def (remove the extra | alternation operator)

^a*$ ⇒ ^a+$ (+ matches 1 or more chars)

^(a)?(b)?(c)?$ ⇒ ^(?!$)(a)?(b)?(c?)$ (the (?!$) negative lookahead fails the match if end of string is at the start of the string)
              or         ⇒ ^(?=.)(a)?(b)?(c?)$ (the (?=.) positive lookahead requires at least a single char, . may match or not line break chars depending on modifiers/regex flavor)

^$|^abc$ ⇒ ^abc$ (remove the ^$ alternative that enables a regex to match an empty string)

^(?:abc|def)?$ ⇒ ^(?:abc|def)$ (remove the ? quantifier that made the (?:abc|def) group optional)

To make \b(?:north|south)?(?:east|west)?\b (that matches north, south, east, west, northeast, northwest, southeast, southwest), the word boundaries must be precised: make the initial word boundary only match start of words by adding (?<!\w) after it, and let the trailing word boundary only match at the end of words by adding (?!\w) after it.

\b(?:north|south)?(?:east|west)?\b ⇒ \b(?<!\w)(?:north|south)?(?:east|west)?\b(?!\w)

Upvotes: 10

anubhava
anubhava

Reputation: 784968

Obviously you need to replace Replace * with +, as + matches 1 or more character. However inside character class you don't to do all that escaping you're doing. Your regex can be simplified to:

^([0-9()\/+ -]+)$

Upvotes: 1

Jan Wy
Jan Wy

Reputation: 1569

You can either use + or the {min, max} Syntax:

^[0-9\(\)\/\+ \-]{1,}$

or 

^[0-9\(\)\/\+ \-]+$

By the way: this is a great source for learning regular expressions (and it's fun): http://regexone.com/

Upvotes: 1

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