QGIS Regular Expression – How to Mask a Dot (Point) in String Replacement

qgisregular expressionreplacestring

In QGIS, I have a string with the pattern (1 character)(1 character)(dot)(number with 1 or more digits).

How can I use regexp_replace() to get the number only?

I tried regexp_replace('ab.27',('(\\D\\D(.))(\d)'),('\2')), but output is '?27', where I want '27'.

How to mask the dot?

Edit

As I have a longer string then just what I presented here for sake of shortness as 'ab.27' and this longer string might also contain digits, I must be able to identify the pattern mentioned above.

So the strings might look as (-> for what I want to achieve):

354ab.27 -> 27
ab.27fg87 -> 27

Best Answer

You can search for any char (.*), followed by a dot (\\.), followed by a number that we capture ((\\d+)) followed by any char. Replace with the captured number.

regexp_replace('354ab.27xyz789','.*\\.(\\d+).*','\\1')

or you can search for a dot followed by a number that we capture

regexp_matches('354ab8.27xyz789','\\.(\\d+)')

Original answer for a pattern of (1 character)(1 character)(dot)(number with 1 or more digits):

Instead of searching for the exact pattern, you can search for what is not a number and replace it with an empty string:

regexp_replace('ab.27','[^0-9]','')

or you can look for numbers only

regexp_matches('ab.27','(\\d+)')

But if you are always having the same pattern, you can just take all chars but the first 3 ones:

right('ab.27',length('ab.27')-3)

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[GIS] What regular expression engine does QGIS use

QGIS, uses QT's QRegularExpression. which is itself just PCRE.

QRegularExpression implements Perl-compatible regular expressions. It fully supports Unicode. For an overview of the regular expression syntax supported by QRegularExpression, please refer to the aforementioned pcrepattern(3) man page. A regular expression is made up of two things: a pattern string and a set of pattern options that change the meaning of the pattern string.

You can see the use of it in src/core/qgsexpression.cpp

Note one Gotcha with the QT C++ implementation is that you have to escape \,

Note that due to C++ literal strings rules, you must escape all backslashes inside the pattern string with another backslash:

// matches two digits followed by a space and a word
QRegularExpression re("\\d\\d \\w+");

// matches a backslash
QRegularExpression re2("\\\\");

Another but very minor non-gotcha, is

QRegularExpression does not support all the features available in Perl-compatible regular expressions. The most notable one is the fact that duplicated names for capturing groups are not supported, and using them can lead to undefined behaviour.

QGIS – Removing Part of String from the Right

You're missing the . after the E; in other words, the token for a character following the E:

regexp_replace(string, 'E.*', '')

There are a lot of ways to make the pattern more surgical. You could use \d if you want to match an E and any digits that follow it:

regexp_replace(string, 'E\d*', '')

You can also use + rather than * to ensure you only match to literal E that are actually followed by at least one digit:

regexp_replace(string, 'E\d+', '')

Just depends on how much variation there is in your strings.

P.S. It also wouldn't surprise me if you're saving the results in a float, rather than a string field. Might explain the case where you get 1.000 as your result

Best Answer

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[GIS] What regular expression engine does QGIS use

QGIS – Removing Part of String from the Right

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