Preceded-By RegEx : negative lookbehind (?<=x)

• (?<=x) means "only if it has "x" before this point"
• (?=x) means "only if it has "x" after"
• Neither will be part of the match

/(?<=[-+:{ ])0(?=\.[0-9])/su

-0.1 = true : 0
+0.4 = true : 0
:0.8em = true : 0
{0.345 = true : 0
 0.234vw = true : 0
0.234vw = false
+0.px = false
+0. 123 = false
:10.8em = false