Regular expression for matching a character except the first occurrence

Question

How to match character in the text except the first occurrence?

Eg:

98C546CC456C67 should match 98C546CC456C67

What do you want to do with the matches? Remove them, replace them, count them...? — Tim Pietzcker, Jul 15 '14 at 11:15
Sorry my solution wouldn't work then. I will leave it anyway in case someone finds this question and uses a regex flavor that supports it. — Stilgar, Jul 15 '14 at 11:20
@Tim - I just want to know whether it is possible in RegEx or not. If not, I'll go with other ways. — kriznaraj, Jul 15 '14 at 11:21

score 1 · Answer 1 · edited May 23 '17 at 10:32

This problem is a classic case of the technique explained in this question to "regex-match a pattern, excluding..."

We can solve it with a beautifully-simple regex:

^[^C]*C|(C)

The left side of the alternation | matches the beginning of the string up to the first C. We will ignore this match. The right side matches and captures C to Group 1, and we know they are the right ones because they were not matched by the expression on the left.

This program shows how to use the regex (see the results at the bottom of the online demo):

var subject = '98C546CC456C67';
var regex = /^[^C]*C|(C)/g;
var group1Caps = [];
var match = regex.exec(subject);

// put Group 1 captures in an array
while (match != null) {
    if( match[1] != null ) group1Caps.push(match[1]);
    match = regex.exec(subject);
}

document.write("<br>*** Matches ***<br>");
if (group1Caps.length > 0) {
   for (key in group1Caps) document.write(group1Caps[key],"<br>");
   }

Reference

score 1 · Accepted Answer · answered Jul 15 '14 at 11:14

1

Unfortunately, JavaScript's regex engine is severely limited. You can't do that in a single regex. The best solution probably would be to do

txt = subject.match(/[A-Z]/ig);  // or /[A-Z]+/ig, if CC should be a single match

and discard the first match.

answered Jul 15 '14 at 11:14

Tim Pietzcker

328,213
58
503
561

score 0 · Answer 3 · answered Jul 15 '14 at 11:01

In a flavor that supports quantifiers in lookbehind like .NET Regex for example you can use lookbehind to look for preceding characters

foreach(Match m in Regex.Matches("98C546CC456C67", @"(?<=C.*?)C")){
    Console.WriteLine(m.ToString() + " at position " + m.Index);
}

Regular expression for matching a character except the first occurrence

3 Answers3