How to find and replace a pattern string using sed/perl/awk?

I have a file foo.properties with contents like

foo=bar
# another property
test=true
allNames=alpha:.02,beta:0.25,ph:0.03,delta:1.0,gamma:.5

In my script, I need to replace whatever value is against ph (The current value is unknown to the bash script) and change it to 0.5. So the the file should look like

foo=bar
# another property
test=true
allNames=alpha:.02,beta:0.25,ph:0.5,delta:1.0,gamma:.5

I know it can be easily done if the current value is known by using sed "s/\,ph\:0.03\,/\,ph\:0.5\,/" foo.properties But in my case, I have to actually read the contents against allNames and search for the value and then replace within a for loop. Rest all is taken care of but I can't figure out the sed/perl command for this. I tried using sed "s/\,ph\:.*\,/\,ph\:0.5\,/" foo.properties and some variations but it didn't work.

Solution

Works with decimal place or not, or no value, anywhere in the line.

sed -E 's/(^|[^-_[:alnum:]])ph:[0-9]*(.[0-9]+)?/ph:0.5/g'

Or possibly:

sed -E 's/(^|[=,[:space:]])ph:[0-9]+(.[0-9]+)?/ph:0.5/g'

The top one uses "not other naming characters" to describe the character immediately before a name, the bottom one uses delimiter characters (you could add more characters to either). The purpose is to avoid clashing with other_ph or autograph.