Creating a regex pattern with a variable for replacing a dot

Question

I need to replace the decimal separator in a string, and the decimal separators could be a dot . (e.g. English) or a comma , (e.g. German). So I have the variable sep for containing the separator string.

To convert the English-based decimal separator, I do the following replacement, but I got ,dd,dd rather than 120,dd:

var sep = '.';
var numberStr = '120.31';
numberStr = numberStr.replace(new RegExp(sep + '\\d{2}', 'g'), ',dd');
console.log(numberStr);

Does anyone know where I went wrong?

basst314 · Accepted Answer · 2017-09-08 15:01:23Z

4

The dot-character in RegularExpressions matches one single character, regardless of the actual character itself (details depending on the programming language / regex engine / flags in use).

If you want to match a dot, your separator should escape the regex dot-selector character like var sep = '\\.'; to match an actual dot, not 'any single character'.

So your error occurs because in 120.31 the pattern [any character followed by 2 numbers] is found/replaced twice, once for 120 and once for .31, as 1 aswell as . match the regex dot-selector '.'.

For details see the Regex Cheat Sheet

edited Sep 8, 2017 at 15:01

answered Sep 8, 2017 at 14:50

basst314

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raina77ow · Accepted Answer · 2017-09-08 14:51:50Z

1

You need to escape the separator (so that it'll be treated by RegExp engine as is) by prefixing it with \ character:

var escapedSep = sep.replace(/[.?*+^$[\]\\(){}|-]/g, "\\$&");
numberStr = numberStr.replace(new RegExp(escapedSep + '\\d{2}', 'g'), ',dd');

Otherwise . is treated as a RegExp metacharacter, matching on any symbol (except line breaks).

edited Sep 8, 2017 at 14:51

answered Sep 8, 2017 at 14:49

raina77ow

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2 Comments

KarelG Over a year ago

To clarify on this ... (he answered before I could do), the regex ended on .\d{2} which means that the dot matches everything. To escape the dot, you have to add \\ so that a dot is presented, not "matches everything except line breaks". But I should look to basst314's answer. Reason: those additional regex. That's uncalled for.

raina77ow Over a year ago

If you check my original answer, you see that was the first version. But did you think what happens when OP changes a separator?

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