How to represent a fix number of repeats in regular expression?

Is there a better way to represent a fix amount of repeats in a regular expression?

For example, if I just want to match exactly 14 letters/digits, I am using ^\w\w\w\w\w\w\w\w\w\w\w\w\w\w$ which will match a w开发者_如何学Goord like UNL075BE499135 and not match UNL075BE499135AAA is there a handy way to do it? I am currently doing it in Java, but I guess this may apply to other language as well.

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For Java:

Quantifiers documentation

X, exactly n times: X{n}
X, at least n times: X{n,}
X, at least n but not more than m times: X{n,m}

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The finite repetition syntax uses {m,n} in place of star/plus/question mark.

From java.util.regex.Pattern:

X{n}      X, exactly n times
X{n,}     X, at least n times
X{n,m}    X, at least n but not more than m times

All repetition metacharacter have the same precedence, so just like you may need grouping for *, +, and ?, you may also for {n,m}.

• ha* matches e.g. "haaaaaaaa"
• ha{3} matches only "haaa"
• (ha)* matches e.g. "hahahahaha"
• (ha){3} matches only "hahaha"

Also, just like *, +, and ?, you can add the ? and + reluctant and possessive repetition modifiers respectively.

    System.out.println(
        "xxxxx".replaceAll("x{2,3}", "[x]")
    ); "[x][x]"

    System.out.println(
        "xxxxx".replaceAll("x{2,3}?", "[x]")
    ); "[x][x]x"

Essentially anywhere a * is a repetition metacharacter for "zero-or-more", you can use {...} repetition construct. Note that it's not true the other way around: you can use finite repetition in a lookbehind, but you can't use * because Java doesn't officially support infinite-length lookbehind.

References

• regular-expressions.info/Repetition

Related questions

• Difference between .* and .*? for regex
• regex{n,}? == regex{n}?
• Using explicitly numbered repetition instead of question mark, star and plus
  • Addresses the habit of some people of writing a{1}b{0,1} instead of ab?

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^\w{14}$ in Perl and any Perl-style regex.

If you want to learn more about regular expressions - or just need a handy reference - the Wikipedia Entry on Regular Expressions is actually pretty good.

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In Java create the pattern with Pattern p = Pattern.compile("^\\w{14}$"); for further information see the javadoc