confusion about ruby handling of start and end of pattern

In the case below where a parrent is to be extracted from a string via a regular expression

input_string = "02150 ESPOO"
input_string[pattern]

Two very different result emerge whether the pattern regex is bookended (starting and finishing) /^\d{5}$/ or not /\d{5}/

I get the desired result in the latter case, while the former returns nil. Why does the bookeneded regex fail?

Answer

The first expression /^\d{5}$/ starts with ^ (which matches the start of a line) and $ (which matches the end of a line). The whole expression can be described as: Match a string, that

• starts a new line
• contains exactly 5 digits only
• ends the line.

This string doesn't match the string you provided, because the string includes two segments one with 5 digits at the start of the string but then there is a unmatched whitespace and 5 letters before the end of the line.

You may try /^\d{5}/ instead (which matches 5 digits at the start of a line).