Test Cases

This kind of string should match our pattern:

• a string of six (6) and only six (6) digits
• a string of ten (10) and only ten (10) digits

All other strings should not match our pattern.

The Regular Expression Pattern

^\d{6}(?:\d{4})?$

The explanation

1. ^: matches the beginning of the string
2. \d{6} matches exactly six digits.
3. (?:\d{4})? this non-capturing (non-capturing because of ?:) group matches exactly four digits. This group is optional.
4. $: matches the end of the string

So, this regular expression could be read as:
Match a string of exactly six (6) digits which could be optionally followed by a string of exactly four (4) digits.
In conclusion, this regex will match a six (6) digits string or a ten (10) digits string (6 required digits + 4 optional digits).

So, my co-worker wanted to know how to retrieve all digits (and only digits: 0 to 9) following a non-digit character and preceding a pipe “|” character ?

Test Cases

This kind of string should match our pattern:

• test123|another456|more789| (should match three strings: “123″, “456″ and “789″)
• justone1|

All other strings should not match our pattern.

The Regular Expression Pattern

(?<!\d)(\d+)(?=\|)

The explanation

1. (?<!\d): negative lookbehind expression tells to the regex engine that the character preceding our matching pattern must not be a digit. Because I’m using a lookaround which is a “zero-width assertion”, this first expression will not be a part of the matching string
2. (\d+) matches one or more digit character. This group is the only capturing group (our matching string) of the regex.
3. (?=|): positive lookahead expression tells to the regex engine that the character following our matching pattern must not be a pipe “|”. Because I’m using a lookaround which is a “zero-width assertion”, this third expression will not be  a part of the matching string

So, this regular expression could be read as:
Match a string of one or more digit character which is preceded by any character except a digit and which is followed by a pipe “|”.

Test Cases

These strings should not match our pattern:

• /
• an empty string

All other strings should match our pattern.

The Regular Expression Pattern

^/.+|[^/].+$

The explanation

1. ^ matches the beginning of the string
2. /.+ matches a string beginning by a “/” and following by at least one character
3. | OR operator
4. [^/].+ matches a string beginning by any character except “/” and following by at least one character
5. $ matches the end of the string

So, this regular expression could be read as:
Match a string beginning by a “/” and following by at least one character or match a string beginning by any character except “/” and following by at least one character.