PATTERN MATCHING WITH REPLACEMENT OF SNOBOL

PATTERN MATCHING WITH REPLACEMENT OF SNOBOL
Pattern matching identifies a subject substring with a particular trait, specified by the pattern. We used conditional assignment to copy that substring to a variable. Replacement moves in the other direction, letting you alter the substring in the subject. The space occupied by the matching substring may be enlarged or contracted (or removed entirely), leaving adjacent subject characters undisturbed. If the pattern matched the entire subject, replacement behaves like a simple assignment statement.
Replacement appears in a form similar to assignment:
SUBJECT PATTERN = REPLACEMENT
First, the pattern match is attempted on the subject. If it fails, execution of the statement ends immediately, and replacement does not occur. If the match succeeds, any conditional assignments within the pattern are performed. The replacement field is then evaluated, converted to a string, and inserted in the subject in place of the matching substring. If the replacement field is empty, the null string replaces the matched substring, effectively deleting it. Try a few examples with CODE.SNO: ? T = 'MUCH ADO ABOUT NOTHING'
? T 'ADO' = 'FUSS'
Success
? OUTPUT = T
MUCH FUSS ABOUT NOTHING
? T 'NOTHING' =
Success
? OUTPUT = T
MUCH FUSS ABOUT
? 'MASH' 'M' = 'B'
Execution error #8, Variable not present where required
Failure
The first replacement searches for 'ADO' in the subject string, replacing it with 'FUSS'. The second replacement has a null string replacement value, and deletes the matching substring. The last example demonstrates that a variable must be the subject of replacement. Variables can be changed; string literals -- like 'MASH' -- cannot.
The following will replace the 'M' in 'MASH' with a 'B':
? VERB = 'MASH'
? VERB 'M' = 'B'
? OUTPUT = VERB
BASH
If the matched substring appears more than once in the subject, only the first occurrence is changed. The remaining substrings must be found with a program loop. For example, a statement to eliminate all occurrences of the letter 'A' from the subject looks like this:
ALOOP SUBJECT 'A' = :S(ALOOP)
Here ALOOP is the statement label, SUBJECT is some variable containing the subject string, 'A' is the pattern, and the replacement field is empty. If an 'A' is found, it is deleted by replacing it with the null string, and the statement succeeds. The success GOTO branches back to ALOOP, and another search for 'A' is performed. The loop continues until no 'A's remain in the subject, and the pattern match fails. Of course, the pattern and replacement can be as complex as desired.
Simple loops like this can be tried in CODE.SNO by appending a semicolon after the GOTO field. (Semicolon is used with GOTO in CODE.SNO only; you would not use it in normal programs.) Continuing with the previous example:
? VOWEL = ANY('AEIOU')
?VL T VOWEL = '*' :S(VL);
? OUTPUT = T
M*CH F*SS *B**T
Since conditional assignment is performed before replacement, its results are available for use in the replacement field of the same statement. Here's an example of removing the first item from a list, and placing it on the end:
? RB = 'RED,ORANGE,YELLOW,GREEN,BLUE,INDIGO,VIOLET,'
? CYCLE = BREAK(',') . ITEM LEN(1) REM . REST
? RB CYCLE = REST ITEM ','
Success
? OUTPUT = ITEM
RED
? OUTPUT = RB
ORANGE,YELLOW,GREEN,BLUE,INDIGO,VIOLET,RED,
Pattern CYCLE matches the entire subject, placing the first color into ITEM, bypassing the comma with LEN(1), and placing the remainder of the subject into REST. REST and ITEM are then transposed in the replacement field, and stored back into RB.。