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In Java/Android, you can use Format strings to specify how some outlook is expected to look like. These Format strings can be passed to String.format() or Formatter(), for example.

Simple examples are %s or %1$d.

Now I would like to write a RegEx that works in JavaScript and lets you extract Java's format strings from a given text.

My solution would be the following RegEx:

%(([0-9]+\$)?)([,+ (\-#0]*)([0-9]*)(.[0-9]*)((hh|h|l|ll|L|z|j|t)*)(d|i|u|f|F|e|E|g|G|x|X|o|s|S|c|C|a|A|b|B|h|H|p|n|%)

However, this does not even work for the easiest case (%s). But it seems to work for all other (more complex) cases.

What I'm asking here is: Would you say this is a good (correct) RegEx or is anything missing?

My sources for the syntax have been: Android's Formatter class and printf format strings

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up vote 4 down vote accepted

The problem lies in this group: (.[0-9]*)

You expect a % at first then the . in the above group is expecting one of something (not optional). Then (d|i|u|f|F|e|E|g|G|x|X|o|s|S|c|C|a|A|b|B|h|H|p|n|%) is also expecting something.

So basically the only required items in your regex are:
1. %
2. the . in (.[0-9]*)
3. (d|i|u|f|F|e|E|g|G|x|X|o|s|S|c|C|a|A|b|B|h|H|p|n|%)

There isn't room in %s to accommodate the % and both the . and the chunk you are wanting to pull the s.

I'm not sure exactly what you want (.[0-9]*) to be, but making it optional like (.[0-9]*)? would atleast allow for %s to pass.

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Oh yes, of course! :) Thank you! If you look at one of the links in my question, you see that this group is for the precision. The precision is optional, but when specified, it must be preceded by a dot. So (.[0-9]*) must be replaced with (.[0-9]+)?. –  Marco W. Aug 19 '13 at 1:59
 
Nevertheless, it does still match %d%%, for example. I'm not sure if this is correct. It at all, it should be two single format strings. According to the syntax (cf. Wikipedia link), %% could be a valid format string as well. Can someone verify this? The first % is the prefix then and the second % is the type. If this is valid, it should still be counted as two single format strings by the RegEx above. –  Marco W. Aug 19 '13 at 3:14
 
@MarcoW., I don't know the specific rules, but it sounds like there are some things in the last capture group that are dependent on some of the things in the earlier groups being there. To allow for % only if there is nothing else ahead of it than the first %, you could do this: %(([0-9]+\$)?)(?:([,+ (\-#0]*)([0-9]*)(\.[0-9]+)?((hh|h|l|ll|L|z|j|t)*)(d|i|u|f|F|e|E|g|G|x|X|o|s|S|c|‌​C|a|A|b|B|h|H|p|n)|(%)) –  smerny Aug 19 '13 at 10:57
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