I want to create a regexp in emacs that matches exactly 3 digits. For example, I want to match the following:
123
345
789
But not
1234
12
12 23
If I use [0-9]+ I match any single string of digits. I thought [0-9]{3} would work, but when tested in re-builder it doesn't match anything.
If you're entering the regex interactively, and want to use {3}, you need to use backslashes to escape the curly braces. If you don't want to match any part of the longer strings of numbers, use \b to match word boundaries around the numbers. This leaves:
\b[0-9]\{3\}\b
For those wanting more information about \b, see the docs:
matches the empty string, but only at the beginning or end of a word. Thus,
\bfoo\bmatches any occurrence offooas a separate word.\bballs?\bmatchesballorballsas a separate word.\bmatches at the beginning or end of the buffer regardless of what text appears next to it.
If you do want to use this regex from elisp code, as always, you must escape the backslashes one more time. For example:
(highlight-regexp "\\b[0-9]\\{3\\}\\b")
[0-9][0-9][0-9], [0-9]{3} or \d{3} don't work because they also match "1234".
So it depends on what the delimiter is.
If it's in a variable, then you can do ^/[0-9]{3}/$. If it's delimited by whitespace you could do \w+[0-9]{3}\w+
[0-9][0-9][0-9] will match a minimum of 3 numbers, so as Joe mentioned, you have to (at a minimum) include \b or anything else that will delimit the numbers. Probably the most sure-fire method is:
[^0-9][0-9][0-9][0-9][^0-9]
As others point out, you need to match more than just the three digits. Before the digits you have to have either a line-start or something that is not a digit. If emacs supports \D, use it. Otherwise use the set [^0-9].
In a nutshell:
(^|\D)\d{3}(\D|$)
When experimenting with regular expressions in Emacs, I find regex-tool quite useful:
ftp://ftp.newartisans.com/pub/emacs/regex-tool.el
Not an answer (the question is answered already), just a general tip.