Regular expression for replacing numbers doesn't work with 'sed'

Question

Using Linux bash.

I have the following code in a file and I want to change the numeric value (1000) to whatever other value.

file:

//...
if (counter==(1000)) {
// ...

I have tried to use this command:

sed 's/[0-9][0-9]+/9999/g' file.cpp

but nothing changes.

Moreover, I want the change to be permanent. How do I do this?

Answer 1

You are using the + quantifier from the Extended Regular Expression Syntax set, but sed by default uses Basic Regular Expressions. So, you need to turn on ERE by adding the -E flag:

sed -E 's/[0-9][0-9]+/9999/' file.cpp

or use the corresponding BRE construct

sed 's/[0-9]\{2,\}/9999/' file.cpp

which is more portable (as noted by @Stéphane Chazelas).

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Note that this will modify any line that contains two or more consecutive digits by replacing that group with 9999. If you want to ensure that only lines that conform to your example

if (counter==(1000)) {

are affected, I would recommend adding a suitable address specifier to make the matching more specific:

sed '/if (counter==([0-9]\{2,\}))/s/[0-9]\{2,\}/9999/' file.cpp 

That way, the substitution would only be performed on lines that match if (counter==(two-or-more-digit-number)).

The g option doesn't seem necessary because in your input example, there is only one such digit group on the line.

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In order to modify the file directly, most sed implementations now understand the -i flag for "in-place" modifications. Again, as correctly noted by @Stéphane Chazelas, if you use it in combination with other flags, be sure to make the ordering right, as -i expects a suffix for creating a backup file (which may be left empty on GNU sed, but is required in FreeBSD/MacOS implementations):

• sed -i '' -E 's/.../.../' will work on FreeBSD sed
• sed -i -E 's/.../.../' or sed -Ei 's/.../.../' will work on GNU sed
• sed -iE 's/.../.../' will not work as the E would be considered the backup suffix.