Problems with javascript regex: replace exactly this number of letters

Question

I'm having problems to define a regex in javascript. I want to replace format strings in this format (i.e. "dd/mm/yy") by strings in this other format (i.e. "dd/MM/yyyy"). My problem is how to use javascript regex. For instance, how to convert the year? I've been trying to convert with these expressions:

f1 = "dd/mm/yy";
f1 = f1.replace(/[^y](yy)[^y]/g, 'yyyy');
f1 = f1.replace(/[^y](y)[^y]/g, 'yy');

With this I attempted to convert all 'yy' not sorrounded by any 'y' to 'yyyy' and then do the same with alone 'y', but I don't know how to say "yy not sorrounded by any y".

Rob W · Accepted Answer · 2011-10-18 16:27:16Z

1

Group the character before y, and refer back to it using $1. "The character before" can be anything except for y. It's also allowed to have nothing before the y, hence |^.
Prefix your pattern by (?!y), which means "match if the next character does not equal y.

Adding both together:

f1 = "dd/mm/yy";
f1 = f1.replace(/([^y]|y)yy(?!y)/g, '$1yyyy');
f1 = f1.replace(/([^y]|^)y(?!y)/g, '$1yy');

answered Oct 18, 2011 at 16:27

Rob W

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Comments

Vala · Accepted Answer · 2011-10-18 16:22:34Z

0

If you want to match exactly two 'y's use /\by{2}\b/ \b is a word boundary which will include the start of the string, the end of the string, and your '/'s (etc.).

answered Oct 18, 2011 at 16:22

Vala

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2 Comments

Ivan Over a year ago

Exactly, Rob W has pointed a non matching regex using word boundaries.

Vala Over a year ago

The regex matches what's mentioned, dd/mm/yyo strikes me as very contrived as you'd end up with something like 01/01/111 which makes no sense.

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