ECMA-262 → PCRE regex translation for the pattern and patternProperties keywords.

JSON Schema defines pattern as an unanchored ECMA-262 regular expression with Unicode semantics. PCRE (Erlang's :re) differs in ways the translation covers:

• $ anchor — ECMA-262 $ matches only at the end of the input when the pattern is not multiline; PCRE $ also matches before a trailing newline. The reference implementation rewrites unescaped $ outside character classes to an absolute end anchor; so does this module (\z).
• Character classes — \d, \w and \b are ASCII, as in ECMA-262, and are spelled out as ASCII classes: the PCRE of Erlang/OTP 27 and earlier counts Latin-1 letters such as é as word characters. \s is ECMA-262's white space and line terminators (PCRE's Unicode spaces plus U+FEFF), \v the vertical tab alone, and . any character but a line terminator.
• Escapes — \uXXXX (a surrogate pair of them is one code point) and \u{X...} become PCRE's \x{...}.
• Unicode property names — \p{...} and \P{...} take ECMA-262's names. PCRE knows the short General_Category values (L, Lu), scripts (Script=Greek, scx=Grek) and binary properties (Alphabetic, XID_Start), but not the long General_Category values (Letter, Uppercase_Letter), the General_Category=/gc= forms or Assigned; those are rewritten.
• XID properties on older PCRE — Erlang/OTP 28 and later bundle PCRE2, whose XID_Start and XID_Continue are exact. Older PCRE lacks them, so there they expand to their derivation: XID_Start ≈ \p{L}\p{Nl} and the Other_ID_Start characters that stay identifiers under NFKC (℘, ℮, U+1885, U+1886); XID_Continue ≈ that, \p{Mn}\p{Mc}\p{Nd}\p{Pc} and the Other_ID_Continue characters (·, U+0387, U+1369 to U+1371, U+19DA).
• Scripts on older PCRE — older PCRE names a script bare (\p{Greek}), so Script= and sc= are dropped there, and Script_Extensions=/scx= fall back to the script. Only full script names (Greek, not Grek) are known to it.

Matching is search (unanchored), per JSON Schema. A search stops after a million backtracking steps: patterns come from agents and catalogs, and one such as ^(a+)+$ would otherwise run until PCRE's own limit, ten times that, and then report no match.