Javascript regular expression: match anything up until something (if there it exists)

Question

I am new to regular expression and this may be a very easy question (hopefully).

I am trying to use one solution for 3 kinds of string

• "45%", expected result: "45"
• "45", expected result: "45"
• "", expected result: ""

What I am trying (let the string be str):

str.match(/(.*)(?!%*)/i)[1] 

This is in my head would sound like "match any instance of anything up until '%' if it is found, or else just match anything"

In firebug's head, it seems to sound more like "just match anything and completely disregard the negative lookahead". Also to make it lazy - (.*)? - doesn't seem to help.

Let's forget for a second that in this specific situation I am only matching numbers, so a /\d*/ would do. I am trying to understand a general rule so that I can apply it whenever.

Anybody would be so kind to help me out?

Answer 1

How about the simpler

str.match(/[^%]*/i)[0] 

Which means, match zero-or-more character, which is not a %.

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Edit: If need to parse until </a>, then you could parse a sequence pf characters, followed by </a>, then then discard the </a>, which means you should use positive look-ahead instead of negative.

str.match(/.*?(?=<\/a>|$)/i)[0] 

This means: match zero-or-more character lazily, until reaching a </a> or end of string.

Note that *? is a single operator, (.*)? is not the same as .*?.

(And don't parse HTML with a single regex, as usual.)

Answer 2

I think this is what you're looking for:

/(?:(?!%).)*/ 

The . matches any character, but only after the negative lookahead, (?!%), confirms that the character is not %. Note that when the sentinel is a single character like %, you can use a negated character class instead, for example:

/[^%]*/ 

But for a multi-character sentinel like </a>, you have to use the lookahead approach:

/(?:(?!</a>).)*/i 

This is actually saying "Match zero or more characters one at a time, but if the next character turns out to be the beginning of the sequence </a> or </A>, stop without consuming it".