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

Question

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 word 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.

score 105 · Accepted Answer · edited Jun 25 '14 at 09:44

105

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}

edited Jun 25 '14 at 09:44

kajacx

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answered Jul 16 '10 at 02:52

shookster

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That is for any perl-compatible regular expression. – Richard Simões Jul 16 '10 at 02:57
@BipedalShark the 'bound' is defined in POSIX regexp standard. See `man 7 regex` on most *nix systems. Most common regex languages including Perl's derive at some point from POSIX's. – Ven'Tatsu Jul 16 '10 at 18:19
for those who didn't figure out how to do "repeat up to m times", you can use X{0,m} – ihebiheb Mar 08 '19 at 18:20

score 36 · Answer 2 · edited May 23 '17 at 12:02

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

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eldarerathis · Answer 3 · 2010-07-16T03:05:19.767

21

^\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.

edited Jul 16 '10 at 03:05

answered Jul 16 '10 at 02:46

eldarerathis

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score 1 · Answer 4 · answered Jul 16 '10 at 02:58

1

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

answered Jul 16 '10 at 02:58

Jano González

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Or use the short-hand: `"UNL075BE499135".matches("^\\w{14}$");` – Bart Kiers Jul 16 '10 at 06:31

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

4 Answers4

References

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