I'm looking for a regexp that matches any line that contains 'B', 'R', 'A' and 'S' (in any order) at the start. It would match all the following lines, except the last two.

BRASIL
BSRAIL
BARSILERO
BRASILERA
BRASILEÑA
BRASILEÑO
BARBADOS
BOSNIA AND HERZEGOVINA

I tried the following:

^(B|R|A|S){4}.*$
^(?=.*B)(?=.*R)(?=.*A)(?=.*S).*$
^(?=.{4})(?=.*B)(?=.*R)(?=.*A)(?=.*S).*$
^(?=.*B)(?=.*R)(?=.*A)(?=.*S){4}.*$
^(?=.*B){1}(?=.*R){1}(?=.*A){1}(?=.*S){1}.*$

CodePudding user response：

You can use

^([BRAS])(?!\1)([BRAS])(?!\1|\2)([BRAS])(?!\1|\2|\3)([BRAS])

See the regex demo. Details:

• ^ - start of string
• ([BRAS]) - Group 1: B, R, A or S
• (?!\1)([BRAS]) - Group 2: B, R, A or S but not as in Group 1
• (?!\1|\2)([BRAS]) - Group 3: B, R, A or S but not as in Group 1 and 2
• (?!\1|\2|\3)([BRAS]) - Group 4: B, R, A or S but not as in Group 1, 2 and 3.

Note that when using the pattern with MySQL v.8, you need to double the backslashes:

"^([BRAS])(?!\\1)([BRAS])(?!\\1|\\2)([BRAS])(?!\\1|\\2|\\3)([BRAS])"

CodePudding user response：

There are only 24 permutations :)

^(ABRS|BARS|RABS|ARBS|BRAS|RBAS|RBSA|BRSA|SRBA|RSBA|BSRA|SBRA|SARB|ASRB|RSAB|SRAB|ARSB|RASB|BASR|ABSR|SBAR|BSAR|ASBR|SABR)

You can shorten it a bit by grouping pairs of two:

^((AB|BA)(RS|SR)|(AR|RA)(BS|SB)|(AS|SA)(BR|RB)|(BR|RB)(AS|SA)|(BS|SB)(AR|RA)|(RS|SR)(AB|BA))

Each double pair matches 4 inputs, e.g., (AB|BA)(RS|SR) can match:

ABRS
ABSR
BARS
BASR