1515 
.sp 
.sp 
1516 
If the text between the parentheses consists of a sequence of digits, the 
If the text between the parentheses consists of a sequence of digits, the 
1517 
condition is true if the capturing subpattern of that number has previously 
condition is true if the capturing subpattern of that number has previously 
1518 
matched. 
matched. An alternative notation is to precede the digits with a plus or minus 
1519 

sign. In this case, the subpattern number is relative rather than absolute. 
1520 

The most recently opened parentheses can be referenced by (?(1), the next most 
1521 

recent by (?(2), and so on. In looping constructs it can also make sense to 
1522 

refer to subsequent groups with constructs such as (?(+2). 
1523 
.P 
.P 
1524 
Consider the following pattern, which contains nonsignificant white space to 
Consider the following pattern, which contains nonsignificant white space to 
1525 
make it more readable (assume the PCRE_EXTENDED option) and to divide it into 
make it more readable (assume the PCRE_EXTENDED option) and to divide it into 
1536 
parenthesis is required. Otherwise, since nopattern is not present, the 
parenthesis is required. Otherwise, since nopattern is not present, the 
1537 
subpattern matches nothing. In other words, this pattern matches a sequence of 
subpattern matches nothing. In other words, this pattern matches a sequence of 
1538 
nonparentheses, optionally enclosed in parentheses. 
nonparentheses, optionally enclosed in parentheses. 
1539 

.P 
1540 

If you were embedding this pattern in a larger one, you could use a relative 
1541 

reference: 
1542 

.sp 
1543 

...other stuff... ( \e( )? [^()]+ (?(1) \e) ) ... 
1544 

.sp 
1545 

This makes the fragment independent of the parentheses in the larger pattern. 
1546 
. 
. 
1547 
.SS "Checking for a used subpattern by name" 
.SS "Checking for a used subpattern by name" 
1548 
.rs 
.rs 
1853 
.rs 
.rs 
1854 
.sp 
.sp 
1855 
.nf 
.nf 
1856 
Last updated: 06 March 2007 
Last updated: 09 May 2007 
1857 
Copyright (c) 19972007 University of Cambridge. 
Copyright (c) 19972007 University of Cambridge. 
1858 
.fi 
.fi 